Because that's what Hasbro's done with their new Metroplex.
Not that I'm saying the old Metroplex was perfect. I mean, look at all the 'fortress' Transformers from the 80s.
- Scorponok: cool scorpion mode, pretty good robot mode, terrible, terrible base mode.
- Trypticon: cool dinosaur mode, terrible, terrible base mode.
- Fortress Maximus: decent (if blocky) robot mode, iffy ship mode, surprisingly not too bad base mode.
- Metroplex: decent robot mode, ok vehicle mode and hmm... could do better base mode.
Notice a trend?
Super-dooper-giant Metroplex takes the crown from Fort Max as being the Biggest Transformer Ever (in height, if not bulk), but that's about it prize-wise.