Why are sidecharacters best girls?
because OP the faggot have shit tastes.
>Why are sidecharacters best girls?
Because their personality isn't expanded on whatsoever and they have no character traits besides very simplistic moods and you are able to project your own desires onto those vapid shells easily.
Because, in most good works, primary female characters are written for female audiences to relate to. This means that, if you want the sexy female who is destined to please, she's generally going to be one of the supporting cast.
The high school protagonist will always choose the nerd with a heart of gold over the cheerful cheerleader with a nice body. The action lead will go for the action heroine over the sexpot. It's just the way that it is.
Her character is less likely to be defined by their love of the MC. She also tends to have simpler personality which allows one to imprint their own ideas on her making her the ideal waifu.
We need more delinquent females.
Because the author usually doesn't have the time/space to ruin them, so they become best-by-default.
Or to rephrase it: Even a pile of dirt can look good when placed besides a pile of shit.
Because best girls aren't meant to win, you can try self insert into the MC all you want but I can't bar a few exceptions nor do I share their taste.