There's another reason too. For a builder, custom building is often a great opportunity to build a name - because it gives the builder access to more land to build on (something that otherwise would have
cost them money) and also often to something that someone else is going to promote, meaning their build will be more noticed by others. They can say, "I built that building," and others can see it to, because the building is right there.
But for scripting, that just doesn't work so well. If someone clicks and passes through a complicated door or portal or other in-world interactive animation, the script making it do so is not at all visible. In fact many non-developer users won't distinguish between the build and the script - it's "a door that opens", and the fact that this actually involved two completely different people is something that isn't borne in mind so often.
To be noticed as a scripter you have to make things where the script is in the forefront, so that people do notice it - and typically those are not the things people ask for as custom scripts, because often the reason why they're asking for a custom script is because they've made something they want to be improved by scripting.
People who are pure scripters - that is, they script and don't build - also have a somewhat different relationship with the world to other content creators. SL's "big rewards" for creation are satisfaction, money, standing within the community, and control over the environment; but someone who can't build or texture de facto can't control the environment. Yes, scripting can exercise control over the environment but only in a limited sense - a pure scripter can make a flying plywood cube, but they can't make a bird (and of course I mean "can't" here in terms of lacking the skill or talent, not lacking the tools). Building a community around scripting is also difficult - some people have managed it (look at the scripted collars, for example, or certain brands of casino games or guns) but it's not automatic given a script (the Multi Gadget is a very good and well known scripted item, but does it have a user community?) It's in no way as natural as it is for a builder who can build an environment that people can actually meet each other inside.
And so for a pure scripter, things often tend to come down to personal satisfaction or money. I even have some scripter friends who wound up feeling that way about SL even though they admitted that they had never wanted to see SL that way. And money for a custom job is often a bad deal compared to making a mass-market product that might earn a hundred or more times the sale price. Of course, that means there's lots of scripters targeting the mass-market, which drives down prices. It's not surprising that I've seen "will script for food" signs sold in shops in SL, and even seen someone advised by an experienced user on scripting that they "shouldn't, there's always some rube who'll do it for you for free". Can you imagine either of those being said about builders or texturers?

Note that this isn't saying you can't get custom scripts done or that doing so is a bad idea (I'm usually happy to do them

) but just to give some understanding of why it might be a bit harder than for other things.