Two reasons—they're a bundle of cliches and caricatures from the 40 year period of 1920-1960 that don't ring authentic to most men; they're male-driven without being masculine.
By male-driven w/o being masculine, we mean they are predicated on the premise that their leading men must evolve to be worthy of love, while their leading women are permitted to remain (more or less) static. This feeds the engine of caricature, because if men must evolve, they must have a fairly cliche body of identifiable traits FROM which to evolve. Essentially, there's SOME need to go from Stanley Kowalski to nearly any Tom Hanks character, and it'd be helpful if they did so in the space of 90-minutes.
While men don't typically identify with the version of 'a man' you find in most rom-coms, most men reject the premise on principle. There is not ONE kind of man, but an entire spectrum of different kinds. Among ourselves, we are constantly evaluating the type of man we're dealing with, in order to make decisions about who to trust in trade, who to respect for leadership, who to accept as friends. Our day to day lives continually deny the premises offered in these films.
But we also don't think the women are real. And more importantly, we don't tend to regard the relationships between men and women in these films as authentic. Men are not, for instance, (as a whole) blubbering inarticulate fools who are terrified of commitment but will walk a fine line between groveling worship and "not putting a woman on an unreal pedestal" if we just get our heads straight. In fact, we suspect that offer is one in which a man has ultimate accountability, but a woman has scarce, flimsy, or duplicitous responsibility. Again, these are male driven, not masculine.
This may explain why there are a few rom-coms that men, not all men but in the aggregate, disproportionately prize, tell their friends about (gingerly, so we're not teased), and watch more than once. Love Actually and Serendipity are two examples. It isn't just the ability to gawk at Keira Knightley or Kate Beckinsale, though it's not a downside for heterosexual men by any reasonable survey of their tastes. These two films permit romance while denying the premise of most rom-coms.
In Serendipity, personal growth is collaborative, mutual, we might dare say equal. Both parties are questioning what lives they want, what they believe, and whether they will choose inertia or aspiration. In principle, it's a partnership, and men like and understand partnerships. A partnership is a joint venture between relative equals.
In Love Actually, there's a bevy of men and women who want various things. Some of each want relationships, some want a good time. Most have no game—any lameness, awkwardness, and foolhardiness is evenly distributed among the genders. Even platonic love is explored and celebrated. The thing between Emma Thompson and Alan Rickman's characters is complex in its depth, simple in the proposition. I won't spoil it for you, but it's a calculation every man makes at some point.
Another point: In both films, 'manly' issues like loyalty, honor, and courage are not omitted in favor of superficial ones like "will he commit or won't he?" "will he grow up or remain a little boy?" — i.e. Man-driven, not masculine thought processes. Men cheer on courage, honor, loyalty in heroes of any gender, and it's deeply limiting to reduce that to a question of whether a man wants or deserves love. Anyone who thinks he doesn't want it, doesn't have a sufficiently robust understanding of manhood. Anyone who thinks he doesn't deserve it either has a sociopathic understanding of love, as a prize, or is mistakenly replacing the man in the film, vicariously, with a sociopath.
Those exist, as we see on the national stage, but it's hard to say anyone, regardless of who they are or what they've done or even whether they understand and can return love, is undeserving of it. That risks multo hubris.
It's fine for rom-coms to be less popular among men than women, and certainly no filmmaker has to cater to anyone's whims. These shows, obviously, are a business as much as they are a form of art. They're entertainment for profit. It's not surprising the assumptions driving many of these films mirror those that drive magazine sales, specifically advice articles like "how to get your man to commit" and "how to please your man", which in turn are loss leaders to sell advertising. It's hard to sell ads when you can't 'nail down' your audience demographic, what they believe (statistically), and what they're asking for (with their spend and ad clicks as data). Of course it's going to be artificial in the same way a breakfast cereal that's a healthy part of a balanced breakfast is simultaneously blather and bullshit.
Rom-coms earn widespread male attention when they don't start out with the notion that men must strive to 'deserve' love, when they don't attribute to men superficial motivations that don't match our experience, when they do present a full range of emotional options for men rather than suggest there are only two types—player and 'good dad', selfish and 'excellent boyfriend accessory'. When the relationships deal with core issues like honor and courage, when they're based on what makes effective partnerships in any walk of life, they get male eyes and male shares.
Perhaps most notably, when they put forth the possibility of an authentic partnership between a man and a woman (most rom-coms are heterosexual), one in which both parties anty up and kick in their share of participation and change, men watch instead of roll their eyes. So ask yourself: does the last rom-com you saw offer that kind of partnership, or a stilted version of it that men would never accept in any male-male partnership in the real world?