It's cheaper for contractors to slap together a bunch of COTS stuff and get good functionality for a "low price" versus building the perfect system from all-in-one designs. The manufacturers aren't going to make military spec all-in-one components because the military isn't buying that much compared to the commercial market so the manufacturers aren't going to invest the time and money into something that isn't going to sell volume. Even if they would make these parts, they are going to cost a lot more than what it would cost to design a slightly more complex, less integrated system. Everyone is better off with more general parts. More components in a system also allow the designer to swap portions out for newer designs as time goes on where as an all-in-one design is going to require a lot of NRE to update.
Military radios are unlike civilian radios in that they have to transmit and receive on a really wide range of frequencies. The Wikipedia article on the AN/PRC-152 handheld radio claims it can receive from 30 to 870 MHz. (As of 2014, they cost $13,000 each, in a tiny production run of 1500 units https://www.militaryaerospace.com/articles/2014/08/air-force...)
The military also wants the ability to change waveforms and protocols on the fly to avoid detection, another thing civilian radios never need.
>The final contract for Rifleman Radios, awarded to Thales and Harris, was for up to $3.9 billion to deliver 193,276 more radios. That amounts to around $20,000 per radio, including accessories and support; in 2014, the military expected the Rifleman Radio to cost about $5,600 per unit. The Manpack radios were expected in 2014 to cost about $72,000; the actual line item for Manpacks under the 2017 budget was $114.9 million for 1,459 radios; that's nearly $78,000 and does not include the maintenance, accessories, and services. That's not exactly the kind of cost savings one would expect from a commodity, standards-based system.