They stream the commercials separately on purpose, because this makes it a whole lot easier for them to track ad impression metrics. Splicing the ad within the same feed is technically quite feasible and indeed almost trivial, it doesn't even require a re-encoding of the entire video. So we can assume that they're avoiding that for a reason.
What it means is adblocker can block the reporting API, but you still get to watch the ad and cost the streaming provider wasting money to splice the ad.
Even if they do it via some sort of chunking, then it's possible to skip chunks easily too (aka, relatively easy to bypass given the amount of effort to implement).
Not to mention it's hard to do caching this way imho.
It'll definitely trouble the non-technical set though.
It's clear where this is heading:
1) Youtube will go after software like yt-dlp to ensure only AlphaGoogle-sanctioned players can play its videos
2) Youtube will encode commercials directly into the videos it streams
Both will come to pass. It's not 'if' but 'when'