Why has YouTube introduced support for multiple audio tracks for videos only now?

It just wasn't a priority. It doesn't impact the bottom line so it didn't get a significant amount of investment

It was not impossible to implement, no. We can only guess as to why it wasn't implemented, but if I had to guess, I'd say it's simply not been prioritized.

Because their product team is incompetent?

Not to come to their defense, I don't like most of what they do, but when you have multiple billion users, every "small change" you make or feature you add is a significant investment in planning, building, and testing.
- As one of the largest companies on the globe, that's not an excuse. They should have the manpower and procedures in place to streamline these tasks.

It's been therd for at least a year, I feel like. My pc default language is Spanish so on some videos it defaults to a dub

I actively had to set my region and langzage to english so YT doesnt auto translate video titles into my native language...
It can't be that hard to implement a switch to opt-out of it, can it?

A couple reasons, I think:

AI dubbing: this makes it way easier for YouTube to add secondary dubbed tracks to videos in multiple languages. Based on the Google push to add AI into everything, including creating AI related OKR's, that's probably a primary driver. Multiple audio tracks is just needed infrastructure to add AI dubbing.
Audio description: Google is fighting enough antitrust related legal battles right now. The fact that YouTube doesn't support audio description for those of us who are blind has been an issue for a long time, and now that basically every other video streaming service supports it, I suspect they're starting to feel increased pressure to get on board. Once again, multiple audio tracks is needed infrastructure for offering audio description.

Wdym "only now"? It's been a thing for like a year now

Since I haven't seen any of the comments mention this yet...

I think the big reason is storage/bandwidth.

Digital audio is an interesting form of media because the size of an audio file is determined almost entirely by 1) how long it is and 2) the bitrate/quality and has a lot less to do with what the actual content is. Therefore, an audio track of a video that contains dialogue and music is pretty much the same size as one that only contains music. So, if you were to, for example, separate dialogue and music of a video into two tracks to allow a user adjust the volume of either independently of the other (an amazing user experience IMO) the storage size (and bandwidth usage) of the audio virtually doubles despite no "additional" content being added.

Multiple audio track is actually something I've wanted for forever, especially for watching stream on Twitch. But I think it's a pretty hefty burden to place on the service, especially if a lot of people aren't even going to use or notice it.

Because google has been doing the bare minimum for it's users in the past decade.

I mean just look at the homepage of google products(except for google.com). They never autofocus on the search bar, why is that?

Just my negative opinion that is based on feeling and not research.