And yes there are packages to install to have the same result, but it should just work out of the box.
Same answer as every other optional package - space. OpenWRT targets very small (in terms of RAM and storage) targets all the way up to full-blown PCs and nearly everything in between. If it's not useful in the general case, it doesn't go into a base build.
OpenWrt uses dnsmasq for name resolution by default, maybe you should direct your question to the dnsmasq developers.
You should be able to use https://openwrt.org/docs/guide-developer/uci-defaults to generate the settings you need, and apply them directly when the router boots, when used in the online image builder.
https://firmware-selector.openwrt.org/
Add any additional packages you need, too.
I understand that.
However, since openwrt is focused on security and stuff, maybe it should be build in.
Dns is a serious thing too, so it needs to go over https/tls right?
I do agree of the "space" problem for some systems, more packages means more file storage, that can cause problems yes.
There is considerable industry debate over whether encrypted DNS is more help or harm.
But DoT and DoH are not security solutions, their intention is to improve privacy. DNSSEC (orthogonal to transport mechanism) is the attempt to improve security, via elimination of man-in-the-middle attacks and such.
Privacy partially overlaps with security and, depending on your threat and trust model, it can also provide data integrity that is essential part of security.
