It’s not just SQL, it’s frequently the OS. Corporate tools don’t support the new OS so you install the “supported” OS, which is now several years old, and which only supports the next version or two of SQL Server. Microsoft also didn’t help things with 2012R2, which was 2 years later but had the same EOL as 2012.
And yes, you can set compatibility level on the database, but there are still edge cases where the engine version matters. And the business prioritizes, but upgrades are lower on the list than money-making features.