Hi
External antennas on routers remind me of the move years ago to 2G and digital mobile phones that used internal antennas, where because the customer expected an aerial, they added fake ones.
Given that virtually every device we have that connects to the router has an internal antenna, it seems odd to worry about the router having internal ones.
Yes if external you can move them about a bit and favour perhaps one direction slightly over the other, but given all the beam forming tricks used now, having more precisely arranged and fixed internal antennas is probably better, hence the proliferation of them, or course it's cheaper as well.
External antennas on routers are little more than a stripped but of of coax wire, whereas internal antennas can be more elaborate and specialised for the job.
Regards
Phil