I think you’d be surprised actually – of the VW forums I’ve been on, changing suspension/gearboxes/engines, turbo conversions, etc, was a lot more common than you’d think.
Besides, you can’t really compare car mechanics and bike mechanics anyway. The former undergoes usually a 3-4 year apprenticeship and works in a garage equipped with tens (possibly hundreds) of thousands of pounds worth of tools and equipment. Whereas you can reach the highest level of Cytech training in under a month and fully equip a bike workshop for pretty much every job for £10k.
Add to the fact that to do most jobs on a car properly you’ll need a ramp (where is your average person going to put one of those?) and it’s dirty, physically tough, sweary work. Change brake pads and discs on a car? Bank on seized bolts, filth and grime, and it’ll take at least half a day off you’re lucky. I’ll pay someone to do that to just not have the hassle. Pads and discs on a bike? Basic tools, under an hour, and I can do it in my kitchen in the warm and dry. Hell, you can strip, clean and rebuild an entire bike in a weekend. Doing basic maintenance and repair on a bike is just not that difficult, expensive, or time-consuming, sorry.
Even so, the average car mechanic salary is only around £20-22k, so why are people surprised when a bike mechanic isn’t earning higher?