"Modular radio" is indeed possible. Almost all GOOD RF test equipment
and professional grade receivers (Watkins Johnson, Racal etc) are
modular in that each section is a tray or block with a 50 ohm
connectorized input and output. But each module costs more than any
consumer radio.

The 10.7 IF module for the IFR 1200 series is basically a fixed
frequency single conversion superhet that has a parts cost of about
thirty dollars, fifteen of which are the connectors and the metal tray
and pan. Last I heard if you were so unfortunate as to need to buy one
it was well in four figures. It is simpler than any AM/FM pocket
'transistor radio' you can get at Radio SHack and contains no ASICs,
no microprocessor, and no custom coils or hybrids. All the miniature
IF cans are Coilcraft catalog parts.

By contrast the total profit in the notebook PC I am typing this on
is probably less than a hundred dollars and that includes that made by
the silicon makers for the chips which constitute nine figure
development budgets. The IF module has a board that could be laid out
in twenty minutes by any competent OrCad operator from a netlist. 10.7
MHz and 455 kHz are trivial to lay out for. The single layer board
probably costs three dollars apiece. he bare board fab in thei
notebook's motherboard is probably considerably more and probably has
eight to twelve layers.

The difference? Several Volume is one. Competition is another.

Very few people are even INTERESTED in radio outside the broadcast
receiver in their car and the various wireless digital gizmos they
own. The market is tiny. And that there is tends to be governments
and such, so the businesses that cater to it are spoiled rotten.