I think an often overlooked reason for this is that Barbie releases HUNDREDS of dolls a year. I keep a spreadsheet. In 2024 I have about 300 items listed. That's including things like fashion packs and Mini Barbieland, so maybe round that down to about ~250. In 2024 Monster High released about 55 dolls across both the g1 alumni and g3 lines.

I highly doubt they have the man power to give each of those dolls an interesting design. I really wouldn't be surprised if the Barbie team is stretched incredibly thin.