This is by design.
SQL is designed to be the only thing on the server. It will consume what you tell it to consume. So you said to consume 24GB which is just a bit less than 80% of the 32GB available. Then the OS and WebApps will need some allocation too, so not surprised to see your seeing a >80% usage.
If it where me I would move everything that's not SQL related to their own servers, and then increase the RAM allocation to 28GB and leave it be.
Hopefully you have capped the application pools in IIS or whatever web software your using to use no more than 4GB total across them, to ensure they can't impact SQL.
Mixing services (Web/SQL) could be seen as a security risk in some organisations.
As a side and a starter for 10, I would grab a copy of the Accidental DBA guide and take a look at chapter 4 for memory management. Some good topics there to mull over to see if you need to do further memory management.