A lot of industries are semi-forced into it. Let me give you an example I know of first-hand. Modern SAP stacks support 3 operating systems. Windows Server, RHEL, and SuSE.

You're probably thinking to yourself: "but rhel is just regular linux, surely you can install it on anything if you have the appropriate dependencies, I'll bet it even just works on rhel-compatibles like rocky, alma, or centos stream!"

And you would be ~sort of~ right, but wrong in the most dystopian way possible. The installer itself does hardcoded checks for "compatible" operating systems, using /etc/os-release and a few other common system files. Spoofing those to rhel 8.5 or whatever is easy enough, but the one that really gets you is a dependency for compat-glibc-X.Y-ZZZZ.x86_64. This "glibc compatibility library" is conveniently only accessible via a super special redhat repository granted by a super special sap license (which is like ~$2,000/year/cpu). Looking at the redhat sources it is actually just a bog-standard semi-modern glibc compile with nothing special. The only other thing you get with this license as far as I can tell is another metapackage that installs dependencies, and makes a few kernel tweaks recommended by SAP.

So you can install it on alma/rocky by impersonating rhel in /etc/os-release, and then compiling a version of glibc and linking it in a special hardcoded location, but SAP/Redhat put as many roadblocks in your way as possible to do this. It took me weeks of reverse-engineering the installer to get our farm off of the ~100k/yr that redhat wanted to charge us for essentially:

./configure --enable-bootstrap --enable-languages=c,c++,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --enable-plugin --with-linker-hash-style=gnu --enable-initfini-array --disable-libquadmath --disable-libsanitizer --disable-libvtv --disable-libgomp --disable-libitm --disable-libssp --disable-libatomic --disable-libcilkrts --without-isl --disable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 9.1.1 20190605 (Red Hat 9.1.1-2) (GCC)

definitely worth $100,000/yr... much capitalism, many line go up

RHEL is subscription based. Not just support anymore. Also for product.