Because like Excel and some other environments, SQL Server enforces a "strong" definition of functions (the one you should have learned in college) that a Function returns a value and has no side-effects. This is as opposed to the "C-culture" view of functions, that cannot tell the difference between a function and a subroutine, and wherein the whole point of calling functions is to cause side-effects.

And the SQL parser, optimizer and data engine rely heavily on the assumption that functions cannot cause changes in the database while they are being invoked. Almost all of the restrictions in UDF revolve around insuring that this is the case.