So let me get this straight...
Adam Sandler is a less credible actor than De Niro because he is typecast into his own movies, for which gain fantastic cult following?
And a man/actor can only be scrutinised on the sum of his work, but if you pick 9 films of a specific genre (this case being outlandish comedy) out of the many, many others he's been featured in then it's perfectly fine.
I'm not saying that Adam Sandler doesn't play himself a lot in movies, he's admitted it many times, but to say that his work is not credible in the least for it, well that's just unfair.
Let's further analyse the example movies here.
So you've basically named all of the earlier movies in Sandler's repertoire, the ones where he plays the some-what loveable moron with a tendency to lash out in rage.
And then two other movies, Little Nicky and Waterboy.
Where he seems to play an unlearned nice-guy.
Now, you're asking why there is no established or 'real' character present in these examples.
I would go as far as to say that, oh it might be because they're characters suited to the style of outlandish comedy I mean did you somehow forget that Little Nicky was about the son of the devil? Realism? Oh wait it's not present from the start!
Waterboy, a movie where in a guy who was sheltered all his life from education and the outside world by his overbearing mother who lives in a swamp working as a 30 year old waterboy for a local college suddenly finds that his anger-- OH GOD why am I describing this, we know it's all in the realm of fantasy by this point...
Adam Sandler has made the majority of his career making movies about the most unlikely of characters, finding themselves in just as unlikely scenarios... The main thing to take away from this is regardless of whether it's your cup of tea (which you've made painfully clear it isn't, well done, job done.) to a particular audience he brings these characters alive.
I mean let's pick out a movie that De Niro wasn't very good in say, 'Meet the parents' or 'meet the fockers' ( a movie in which De Niro AND Hoffman lose credibility in my eyes)
Simple fact is, I acknowledge that not every actor today is capable of believably portraying a character all of the time, some can't even do it some of the time. (I'm looking at YOU Christian Bale...) and yeah,the sole purpose of movies isn't entertainment... Wait, what am I saying? Yes it fucking is!
Sure you can get arty-flicks or movies with a message attached or something that pushes the boundaries of yadda yadda yadda... But when it comes down to it, movies are about two things, money and entertainment.
And believe me, I studied this stuff for two years to become a Teaching assistant for a Media Production Course.
Adam Sandler manages entertainment in his own way, just like Robert De Niro...