Maybe they have to pay them more? I know typically a player's salary is close to 10% of what they make while in the majors. Unless there is a special September rule, that will likely be the main reason.
Exactly. As soon as they get up here, they get the MLB daily minimum.... which just the per diem alone would probably be close to what they are making now. Then you factor in hotel costs, charter flight costs, equipment/uniform costs... and you can see why teams don't just call up guys who only maybe would contribute. Fontana won't be a better defensive option unless there's an injury to one of the other infielders. Same goes for Applin, unless they continue to give Springer some DH time which means he'd sub in for Hernandez.
When Marwin & Rasmus were hurt, we sure had use for both as potential defensive subs. Even now, if we pinch hit for Jake or Teoscar, Aplin would be a good defensive replacement in the OF.
I suspect it is more for logistics than money. There just is only so much room in dugout and locker rooms for guys to be comfortable. The additional bodies would get in the way during batting practice and fielding drills. Just more trouble than it would be worth, IMO. Don't want guys to annoy the "veterans".