You must decide that it's necessary. As an engineer, you are the most qualified individual to determine if something is technically necessary. Just like you decided that proper indentation, unit tests, refactors, etc. are important, so must we decide that other elements are.
That's exactly the point and you're essentially nitpicking (with anecdata) the fact that he pinned the problem on developers instead of product managers. Regardless of who is making the decision in companies, the end result is pretty much the same.
It brings the site closer to recommended accessibility guidelines, making it easier for e.g. people who have vision loss to navigate. It also reduces our potential legal liability to such users for not meeting accessibility requirements.
Me: Hey, I'd like to add a work-item to our current sprint that reworks semantic attributes into our site.
Them: What would that do?
Me: It improves the underlying architecture making the markup more usable and extensible.
Them: How does that benefit the users?
Me: Over time it will reduce the TTM for features and allows us to adopt a common standard others use on the web.
Them: So there's no UI and it won't affect their workflow?
Me: Well, no.
Them: Then why would you suggest it? We don't have the budget to add meaningless development tasks.
I might be biased towards thinking there are plenty of developers and engineers out there thinking about this stuff but we don't always get the final say in what is done. Also, a lot of enterprise applications were and continue to be around before many of these standards. It's a struggle to improve what doesn't directly add to the bottom line.