Why do people call these trucks money pits/question about 4X4 conversion

[Jarvis-69 GMC 1500CK]

I keep seeing people call these trucks money pits. Just wondering if things seem cheap to me due to my previous experience with vehicles or if there is something that I am missing.

I come from the land of 90's and newer vehicles/Cadillac where a single relay can be 400$, costed me 600$ to get a distributor for the north star V8!. I have yet to find a part for my 69 GMC(excluding whole engines /trannies/whole wiring harnesses) for over 200$.(my father just replaced the turbo on his 06 f250 for more then i paid for my truck!)

With that said,I might be missing something as far as budget is concerned(I am pretty new when it comes to this sort of stuff, so it is possible that I am completely over looking something!)

Since we are on the talk of $$! I am looking to convert my 69 over to 4X4 either via just getting the parts(difficult to implement, I know) or using a donor 4X4 frame and wonder what sort of part budget I am looking at for donor/parts.(I use my uncles hot rod shop so got a billion tools and I do not consider my time a cost as I absolutely love doing it)

(My body/running gear/frame is in decent shape,I do not intend on lifting or anything like that, I live very rurally and getting real tired getting stuck where even my cadi rolls right through!)

if I do use the donor method, can I use the frames from long suburban/blazer(I have a long box)? I was kind of thinking of picking up a 4X4 SUV, swapping it out and giving the newly 2WD blazer/burb to my mother who has no need for 4X4 And what years would be fairly "easy" to swap out for 69 GMC1500.