Inserting the assertions by hand is annoying and error prone - even if you want the assertions to be checked at runtime its still very helpful if the programming language inserts the type checking automatically for you.

Another thing is that assertions can only check primitive type. To check if a function pointer or object respects an interface you need to add an extra wrapper around it to check all its return values (and this is so annoying to do by hand that noone bothers to do it)