im ot sure the right term is "less serious".. perhaps less intense? less demanding? less "risk to investment of time and money"?
Example:
If you get a radio hit, and crash your truck into a tree, you are out a couple of hours time, and maybe 50-100 in repairs? If I am in a low pass @ 180-200mph with my Turbine F-104, and I get a glitch, i slam into the runway, a great big fireball ensues (yes, turbine RCs explode and burn just like the full scale), and all of a sudden my wallet cries to the tune of 6000-7000 bucks, plus the hundreds of hours invested in building it.
I am in no way saying one is "better" than the other, just the RC aircraft definitely require a higher level of dedication, skill, and monentary investment... Cars certainly have their place as well, but are simply easier, less risky, and less demanding of the operator.