I wouldn't dare to compare ratings for het romance with ratings for gay romance, as I haven't watched a pure het romance for ages.
But I do notice that dramas, regardless of genre, are subjective to misleading ratings.
Sometimes due to fan wars, whether mass upvoting or mass downvoting. Sometimes just because -- for example, Japanese titles generally are reated a few points below Korean titles.

Roxanne_aishi:
when in reality most of them are very bad
I'd be interested to hear which BLs you've watched to come to this conclusion.
I took a look at your watchlist and couldn't find any in either your Completed, your Dropped or your On-Hold lists. On your Plan to Watch I found exactly two BLs (My Love Miy-Up (Japan) and Ossan's Love Returns) and one Chinese censored BL (Winter Begonia). I might have overlooked a few. (Yes, I scrolled through all 2358 items on your PTW list.)
I would say there are quite a few BLs of dubious quality, and there are many more mediocre ones, as well as a few outstanding BL dramas.
But really, I can't imagine that it's much different with het romance dramas. (Or GLs. But their sample size would still be too small to make a reliable statement.)
Any kind of "quality" distribution usually follows a Gaussian Curve.

Roxanne_aishi:
no matter what they do, if it's BL, it's going to get a high rating
Not true. There are BLs with a rating of below 7.

Roxanne_aishi:
And that's why the quality of BL dramas is so low
If we're talking about quality, we should consider that live-action BLs still are a niche product, with a very specific kind of target audience.
a) It has been hard for producers to get funding, Still is, in most countries, epecially in South Korea, except in Thailand, and they have a different kind of problem, with one company dominating the market.
b) Until very recently, viewers have been happy to get live-action BLs at all, let alone high-quality ones. Only in the last two years or so, have we seen an rise in numbers equalling the Cambrian Explosion.
c) Audience demographics skew towards younger viewers who can be easily excited.
d) And with teenaged girls as the target audience, topics in BLs have tended to favour easily digestible school romance. As I said, we are in the midst of something similar to the Cambrian Explosion, so we are currently seeing a diversification of genres and settings.
d) Sex sells.