Sure! But a day only has 24h and you need to work, rest&relax, eat and shop somewhere in there. If a repair job costs me 1h of time or 2h's worth of labour in money, it's not clear that I can afford the 1h of literal time.
Not to mention that a job that it might take a service 1h's worth of labour to do may take me 2-3h, amortized with the learning curve (say, 4-5h first time I try, and eventually down to 1.5-2h after I've done it a few times).
You're right. In the end, it comes down to picking your battles. There's no hard rule for which battles are worth it.
I do my own motorcycle repairs, because I sometimes stray far from the nearest mechanic, and field repairs can be necessary. I also cook a lot, because I have to eat every day and restaurants are expensive.
However, I paid someone to rebuild my forks, and I buy pre-made pie shells. Those things are not worth doing myself. My father rebuilds engines in his garage, but won't change the oil on a slush-covered car. He'll pay someone else to do it.
I guess it depends on how pleasant the job is to you, how often you need it done, and how much it costs to get it done relative to your own salary.
Not to mention that a job that it might take a service 1h's worth of labour to do may take me 2-3h, amortized with the learning curve (say, 4-5h first time I try, and eventually down to 1.5-2h after I've done it a few times).