Probably because they think it's "just sticking letters on the wall". Once they drill their holes and get one or more studs hung up and they wont go any further into the holes and they can't pull them out of the holes, they'll be wishing they had paid someone to install them. Just be careful how much time you spend trying to be helpful.
When I used to offer dimensional letters turnkey (manufacture (via vendor) and install), I dealt with one of the cheapest individuals I've ever met. I sent art showing 3/8" acrylic and he asked if we could use thinner acrylic 1/8" acrylic to cut costs. He asked if he could pick the letters up from the vendor and spray paint them himself to cut costs then bring them to the site and help me install them. He had a handful of other odd requests including borrowing a ladder from me (I had quoted using a lift due to the height). I said no to all of that and we finally agreed that I would simply provide the letters and he would handle the install. I would have bet a weeks pay that he was going to call and ask how to install them. Sure enough, he called a couple of days later. The building was brick and he didn't have a clue where to start. He asked "what's the best way to install these?" I had wasted enough time with him already, so I told him I would email him very detailed written instructions for $300. This was a few years ago. I've driven by the place and the letters were never installed. He probably couldn't get anyone to work with him and I wouldn't doubt if he went out of business before he really even got started. Or maybe he moved and put them up somewhere else.