It's like this for the current Adobe Creative Cloud apps also, and comes as no surprise. Here's what's likely the reason:
Apple only issues security patches for the last 3 operating systems at a time (i.e. 11, 10.15, 10.14). It's been that way for the past 10 years or so.
Time to upgrade, and if your machine doesn't support Mojave, EVEN MORE time to upgrade...
If you don't like upgrading operating systems, you could always switch to Windows 10, which has been out since 2015 and continues to receive patches 😄.