It's almost like the users prioritised their weapon integration requirements and the contractor followed that...

There isn't infinite people and facilities
NB it took 16 years past IOC to integrate the first powered air to ground munition on Eurofighter. Not that I'm sure this is a good measure anyway.
We're also now talking a lot about integration of weapons which didn't even exist as concepts at the time F-35 SDD / Block 3 was contracted - it's not surprising that these are in Block 4 instead.
Obviously it's a terrible idea when MOD actually identifies a plan that reduces near term costs and increases performance and capability.