You're not going to find this answer satisfying, I suspect, but there are two main reasons browsers and big sites (that's what we're talking about) didn't bother to try to make DNSSEC faster:
1. They didn't think that DNSSEC did much in terms of security. I recognize you don't agree with this, but I'm just telling you what the thinking was. 2. Because there is substantial deployment of middleboxes which break DNSSEC, DNSSEC hard-fail by default is infeasible.
As a consequence, the easiest thing to do was just ignore DNSSEC.
You'll notice that they did think that encrypting DNS requests was important, as was protecting them from the local network, and so they put effort into DoH, which also had the benefit of being something you could do quickly and unilaterally.
It's just a very difficult statistic to get around! Whenever you make a claim like this, you're going to have address the fact that basically ~every high-security organization on the Internet has chosen not to adopt the protocol, and there are basically zero stories about how this has bit any of them.