The main reason, as mentioned already is the car needs to be uprev'd to achieve the gains that fitting the plenum can offer, some will adopt the attitude that if the garage has to do the uprev then they may as well do everything. Others clearly want to learn and test themselves a bit by DIY'ing.
I guess "easy to fit" should be gauged by your own skill level, and let's be honest most of us know what we are capable of. I did my own plenum but only after I had read manuals and instruction sets and determined that I was happy to attempt this, it went well and then it went to Abbey for the uprev. Now, compare this to a supercharger installation, for example, I would certainly seek help from someone more experienced than me if I did indeed decide to do this. Take a look at Keyser's thread and darren-b's too and you soon start to realise what some people can achieve. So, the key for me is research before attempting anything, don't take an "it's easy job" as an answer from anybody, it maybe for them but not for you or me.
Just my two penny worth