Those "sensor pods" are shaped like external fuel tanks. They've got that rounded and curved shape, to make them aerodynamic. Which is horrible for stealth. The F35 has to pack all its baggage inside the fuselage, with minimal openings.
A huge part of this question then becomes a tradeoff between stealth and features. You have to gve up some stuff if you want to be stealthy. So far, on the F35, most of those drawbacks have been "bought out" by spending a crapton on working around them. Stealt VTOL for example was a major PITA.
Considering the already absurd cost of the avionics electronics developed for the F35, tacaking on a completely new ground sensor package (and finding a place to PUT it inside the airframe) would have raised the cost quite a bit. Those sensor pods have been a work in progres for the last 15 years, the R&D is already mostly done. You can't compare that to a completely new package. (and you thought the rest of the new F35 had bugs and glitches?)