Because very often it's not up to the developer/architect to pick the technology to work with. Also what seems expensive to you, very often is not so for the customer.
Either you have Enterprise Windows (costs about as much as professional with the CALS), or the software you are running is only local to that server. The Microsoft treadmill is expensive.
If you building some internal business app, then you need CALS.
That also appears to only cover web servers that do not have any login capability. Why on earth would you spend that kind of money to host a static site?
You would license SQL sever on a core based licensing model, which doesn't use CAL's.
If anyone can purchase a login to your website, that's still classed as public. What its means is internal apps require licenses.
[0]https://www.microsoft.com/en-us/cloud-platform/sql-server-pr...
EnterpriseDB which is basically support for PostGres is almost as expensive.
Special licenses for various uses cases is another big plus for FOSS/PostgreSQL. Life is easier when adding database nodes or spinning up a dev/qa database is purely a technical issue.