Google still has to routinely invent new techniques to stop spam. This problem is not solved - it's an ongoing arms race.

It is also a vastly easier problem - spam filters at google scale work because they see the same spam across zillions of accounts. This is not the case for youtube, where a spammer provides them with far fewer samples, and those are across a much wider space.

These problems are barely related.

80-90% of the emails in my "Spam" folder are not spam.

It's worse than no filtering at all, since I still get exposed to the spam, but now I have to check two inboxes instead of just one, and then jump through annoying hoops to view images or access links in the second one.

Gmail has the worst spam filter of any email provider I use today, excluding the one email server I have running on my domain that gets almost no email and has no filter. Or, well, spam filtering is a lesser problem on my server too because it doesn't block nearly all the email that I care about.