a cam or intake & carb swap is wholly within the wheelhouse of the general hotrodding public or shade tree hobby types. whereas gears, unless you're talking about swapping a pig on an 8.75 is a little more involved on a number of different levels: skill, specialty tools, cost or even availability (say you've got a 7.25? what then?).
same for a converter. a lot of people have a fair amount of trepidation about transmission work, don't care for the under car action that takes or have reluctance about laying down $400+ for something that you can't really quantify and "well, it seems to work just fine now".