Except they haven’t really sat on him for months.
Yes, he started out the year sizzling (155 wRC+ over his first 134 PAs), but then he got hurt.
When he returned from injury a month later, his pop was gone, 112 wRC+ over 119 PA from 6/16 to 7/25 with only 2 HR.
Since then he has hit for a 166 wRC+ over 88 PA with 8 HR, so the pop looks to have returned over the last few weeks.
I’d say there was reasonable justification for him not to make the OD roster, not to be called up after only six weeks, not to be called up while injured, and not to be called up after returning from injury with no pop and the trade deadline looming.
If they don’t call him up Sept. 1, the writing will really be on the wall though.