The Dems hate him, even the right leaning ones.  The Reps are scared to be seen with him because the midterms are coming right up.  Besides, the Congressional Baseball Game is a long standing tradition where the two sides can come together to enjoy themselves, share a dog and a beer and everybody in congress is kind of looking forward to it.   Trump would not be welcome in that scene.

Regardless, he will probably show up unannounced because that would be his style - it enhances the focus on him, which is and has always been his primary concern - look at me, I'm [a totally clueless moronic] President now!