I was wondering the same thing not long ago.
I personally think it's because he became so obsessed with themes. I mean on a guest verse or a crazy D12 track he can say what the fuck he likes, doesn't need to be relevant to anything.
So he can just flex his lyrical muscles.
But on his own albums like... With The Eminem Show and Encore, seemed like he was trying to tie them together like the first two. And he loves making his albums like movies etc. So I think he was trying to run a thread through Encore but due to lack of inspiration ended up doing stupid shit too.
Also, I think you got some lazy shit on there because he re-did some material very quickly. "We As Americans", "Bully", "Love You More" and "Ricky Ticky Toc" were all supposed to be on it. Then they leaked and he re-made aload quickly. I'm guessing those would be "Rain Man", "Big Weenie" and "Ass Like That". All the original songs are classics.
He probably also forced himself to be silly on those when he didn't want to because probably had assholes around him telling him the album was too serious. Because think about it, without those three silly songs and... "My 1st Single" - If the original songs were left, Encore would be a very serious ablum. Politically driven, serious, serious, serious.
So I think part of it was the leak and just speculation but possibly people at Interscope urging him to be funny and lighten it up for kids etc. I only just thought of that but that actually makes sense to me. Since were also rumours of Interscope trying to get him to aim Relapse 2 at more black audiences.
But in general he seems to be better when not on his own stuff. Like he can pull out an extra 10% for guest spots and D12 verses. Or like he's holding back on his own... Maybe just to stick to theme.