Why is an Active-Active configuration not recommended by F5?

Simple reason is you can end up running over 50% of your F5's available resource then a failover would create lost connection.

F5 recommendation is to run LTM pairs as Active/Standby to prevent load every going above 100% of a single LTM. However this is only a very general recommendation and depending on your setup and traffic Active/Active is sometimes a valid approach, one I have configured a couple of times before.