mate-desktop-environment-core is a meta-package. That means it doesn't necessarily provide anything useful in and of itself; all it is, really, is a sort of "fake" package which depends on and automatically pulls in other (usually real) packages. Meta-packages exist as a convenience: Instead of explicitly installing a list of 65 (for example) packages that are all related (such as part of a desktop environment), you can simply explicitly install the meta-package and the meta-package will take care of installing everything else.
Of course, metapackages can get in the way, especially if the metapackage maintainer's preferences clash with yours. Generally speaking (notice the emphasis), metapackages are not essential to have installed; as a matter of fact, I don't have mate-desktop-environment-core installed on Debian here. I'm not at an Ubuntu MATE system at the moment, and I can tell you that some metapackages such as ubuntu-mate-desktop are necessary if you want a smooth-sailing upgrade to 22.04; but my advice is if you really don't want the MATE terminal, and you don't care about having the exact 22.04 desktop after an upgrade (chances are, you don't care, since you're replacing the MATE terminal anyway), run (but do not confirm the operation just yet!):
sudo apt remove mate-terminal
Look over the list of packages that would be removed. If all of the packages to be removed are the MATE terminal or are metapackages, then it's probably safe to remove the MATE terminal. (Hint: You can usually find out if a package is a metapackage by looking over the output of the following command:
apt show [package name]
Usually, the short or long description of the package contains the word "metapackage" if the package in question is a metapackage.)
As for the "why" part of the question, chances are the Debian or Ubuntu MATE maintainer of the metapackage decided that the terminal is just too fundamental a component of the MATE desktop to do without. Like I said, I don't have this metapackage installed on Debian. That's one upside to using Debian instead of Ubuntu MATE: You get to choose everything you want from the very start, nothing more and nothing less.