Remember that a digital RX has a lot going on. The end result is some electrical and rf noise.
By having the basic RX located away from the digital circuitry, you can improve the sensitivity by improving the signal to noise ratio.
Then there is a signal blocking issue, due to the various parts of the model, and short antennas.
Anyway, the sensitivity of the modern 2.4ghz R/C receiver is quite good when designed properly, and top grade components are used.
It's also well to note that even so, the sensitivity of one RX to another can vary by a significant amount. In other-words,
a receiver that "just meets" testing requirements may be half as sensitive as one that is at the top end of the scale.
My Background, for whatever it's worth, or not!
Back in the late 1960's, I spent some time in the Navy as a shipboard electronics tech, and often the only on board tech.
In the Navy's usual manner, I was trained in general electronics and radio communications equipment, and then promptly assigned to radar and navigation systems.
(And expected to provide radio communications repair when the other techs could not!)
At that time, a large number of the older radar systems designed in the late 1940's and early 1950's were still in service on the small ships that I was assigned to.
1.5 to about 2.5 Ghz was the general operating frequency range. The low power radar systems had a range of about 20 miles, and the medium power systems
up to about 200 under ideal conditions. The medium power units were sort of a conversion from early antiaircraft radar, in that they did not have an air search antenna
and the appropriate display. This made sense, in that it allowed use of common parts that were available in large numbers, although obsolete for antiaircraft radar use.
We spent a lot of time at sea between Japan, the Philippines, and Vietnam in good weather and bad, and I spent a fair amount of my efforts keeping radars and Loran receivers
going. There are things in the China sea that you really don't want to run into in the middle of the night or in bad weather!
In and around Vietnam, we often made night time firing runs. During the day, we'd navigate visually, and record radar based positions to match the visually confirmed positions.
At night, we'd come back, and use radar to establish the proper firing positions, and fire rockets and 5" gun rounds into designated targets.
Decades later, I ended up spending a major part of my career in of all things, a specialized field that designs and builds equipment intended to jam airborne and ground based radar.