The issue is that they're not commonly used, and even if that changes, the ISPs can roll out harder-to-bypass censorship methods like SNI inspection or IP blocks.
DNSSEC gives you the ability to verify the DNS response. It doesn’t protect against a straight up packet sniffer or ISP tampering, it just allows you to detect that it has happened.
DoT/DoH are better, they will guarantee you receive the response the resolver wanted you to. And this will prevent ISP-level blocks. But the government can just pressure public resolvers to enact the changes at the public resolver level (as they are now doing in certain European countries).
You can use your own recursive, and this will actually circumvent most censorship (but not hijacking).
Hijacking is actually quite rare. ISPs are usually implementing the blocks at their resolver (or the government is mandating that public resolvers do). To actually block things more predictably, SNI is already very prevalent and generally a better ROI (because you need to have a packet sniffer to do either).
Of course you will need to configure your DNS server/client to do local validation for this, and at most it'll prevent you from falling for scams or other domain foolery.
The parent comment is also correct that the best DNSSEC can do for you, in the case where you're not relying on an upstream DNS server for resolution (in which case your ISP can invisibly defeat DNSSEC) is to tell you that a name has been censored.
And, of course, only a tiny fraction of zones on the Internet are signed, and most of them are irrelevant; the signature rate in the Tranco Top 1000 (which includes most popular names in European areas where DNSSEC is enabled by default and security-theatrically keyed by registrars) is below 10%.
DNS-over-HTTPS, on the other hand, does decisively solve this problem --- it allows you to delegate requests to an off-network resolver your ISP doesn't control, and, unlike with DNSSEC, the channel between you and that resolver is end-to-end secure. It also doesn't require anybody to sign their zone, and has never blown up and taken a huge popular site off the Internet for hours at a time, like DNSSEC has.
Whatever else DNSSEC is, it isn't really a solution for the censorship problem.
DNSSEC doesn't prevent censorship, but it does make tampering obvious. Moving the point of trust from my ISP to Cloudflare doesn't solve any problems, Cloudflare still has to comply with national law. DoH is what you use to bypass censorship; DNSSEC is what you use to trust these random DNS servers you find on lists on Github somewhere.
A bit over half the websites I visit use signed zones. All banking and government websites I interact with use it. Foreign websites (especially American ones) don't, but because of the ongoing geopolitical bullshit, American websites are tough to trust even when nobody is meddling with my connection, so I'm not losing much there. That's n=1 and Americans will definitely not benefit because of poor adoption, but that only proves how much different kinds of "normal internet user" there are.