There are 2 kind of main characters: blank sheets and characters properly said. Take Final Fantasy as an example, Final Fantasy 1 is very different from Final Fantasy 7 for this reason. There's so much you can go with blank sheets in term of story, and that's a problem when most of the things you do in your game is read and interactions are based on text rather than visual representations because you are limited to it. Games like Skyrim are easier to have blank sheets because they are more visual than a game in the NES or PS1 was, so they needed ways to make the main character interesting, and thus the story, and this is even more important in visual novels.
But having a character with personality doesn't mean you can't identify with it. If you give the player to input a name, even if they don't input their own, you want something for them to be hooked into, and many many times you see complain about how MCs act like idiots either because they don't have personalities or lack motivation to move the plot forward, which is kinda important when text is prominent in your games.
If you see a character that is cool, you either want to self-insert as that cool character, specially if it has your name, or you are simply interested in seeing how the story ends for said character. My best example is Yuuji from the Grisaia serie.
You said so yourself, a roleplaying game doesn't mean you are necessarily playing yourself, I never said that, but it means YOU, in this context the player, is the one playing, and as a person previously said, they don't want to play as themselves for a variety of reasons, but they want to play themselves, and even though this is not limited by gender in most cases, psychologically this is the path to go, even when you personally have a different taste and preference for female MCs, you can get hooked easily if you are represented and attracted to a "strong" male character. Doesn't have to be very complex, just someone you may like. And it does apply to females, even, like Final Fantasy 6, which is considered one of the best Final Fantasy games, but a lot of people agree that Terra is probably the worst part of FF6, to the point characters like Squall and Cloud are more popular despite their games being considered lesser in some cases and Celes, Locke and Kefka being more popular than Terra herself. In a semi unrelated note: When a side character is better written than your main character, people usually like it better and want more content with said character, as previously mentioned, and that's still valid. Like Golbez in Final Fantasy 4.
I hate the thought of "everything is subjective", because yes, tastes will always exists, but that doesn't change that objectivity should always be worked on. In what aspect? Well, this is about development, isn't it? Let's put 2 japanese developers as example: You have Black Lilith, which is a company that will always make games about rape and the such. If you don't like rape, you avoid Black Lilith. Then you have Moonstone, that develops bishoujo games for women, but they themselves have sub-brands like Moonstone Cherry that develops nukiges. The cool thing with japanese developers is that they have been making these games for so long they know how they should divide themselves, who to focus to, and solely focus on that aspect for a very minor exceptions. In this case is like Alicesoft decided to make Rance a woman for some reason and expect people to be happy with it.
