They need the capability to interface with weapons, even if the weapons don't have motors or warheads.
Avionics has to be there to be able to incorporate seeker behavior.
Weapons integration is always an ongoing process with every fighter back as far an anyone can remember.
Every time a new model or block upgrade is made to Air Intercept Missiles or bombs, they have to be validated by operational test and evaluation units, then have the avionics upgraded across the fleet of the aircraft that will be made compatible with that weapons upgrade.
Even with dumb bombs, they had to be tested and validated at different speeds, flight regimes, stores configurations, and altitudes.
It gets even more complex with AIMs and PGMs. Look at the video of the F-35 testing AIM-9X, where they included barrel roll launches even with separation happening when the aircraft was inverted.
Like every other aircraft out there, the F-35 will never be finished with systems and weapons integration.
The real main difference with the F-35 vs the F-16, for example, when looking at costs, is that all of the ancillary and follow-on systems that ended up on or in the F-16, have some type of equivalent and more integrated into the F-35.
That includes IR and RF detection, tracking, and targeting, as well as advanced self-protection features.
In addition to those, you have low observable surfaces and internals, more sensors, and a radar that is in an entirely different generational leap over what's in the F-16.
That they are able to deliver at the current unit costs should be hailed as a major accomplishment, rather than ridiculed by those who don't know what is under the hood of this aircraft. It's very ambitious across the board.