Besides, being able to fix something yourself gives you a sense of satisfaction that you just can't get from paying someone to do it.
I made a shoe cabinet recently - it had to be of a particular width. I spent about $200 on materials and it took me roughly 8 hours. I could say it cost me $1000 (let's say I get paid at $100/hr). I could also say it cost me nothing, as I might well have spent $200 to entertain myself otherwise. I also do most of my own plumbing - I find it fun.
One thing I noticed after doing DIY for many years: My DIY manual skills are way higher than those of my friends/family who do not do DIY. They find it rather hard to do simple tasks properly, like hammering in a nail, or driving in a screw.