I think there's a bit of a misconception about what motivates a person to post on LEGO Ideas.
As of today, a total of 33,343 projects have been submitted to LEGO Ideas over time. Only 259 have reached the 10k marker and only 37 of them have been approved as LEGO Ideas sets so far. It'd be rather arrogant to create and submit a project assuming you were going to beat out the thousands of other competitors to become one of the very few who have succeeded. Less than 1% of submissions even make it to 10k and only 14% of those have been chosen.
As for me, I enjoy playing with LEGOs. However, being an adult I have few ways to express that in a meaningful way that others will appreciate. Whether or not my idea will pass the approval process, I had fun designing it, I had fun rendering it, I had fun figuring out interesting mechanisms, and I was gratified by the many people who chose to show their appreciation by supporting it. It is a hobby I've managed to share with the world and the creators of a toy I've been enamored with since childhood. The financial incentive certainly helped motivate me in the creation process, but I never really believed it would reach 10k. I just wanted to make the best design I could and watch how far it could go.
LEGO is a toy. It's meant to be played with and LEGO Ideas is a way to give back to the fans who most enjoy their product by giving them a chance to be a part of its creation in designing and choosing what they think is best.
As for what an artist chooses to build, yes many things don't stand a chance a getting approved. However, for me, over analyzing my hobby in terms of market value kills the fun of it. Not to mention that I wouldn't be capable of creating certain types of projects. I've never made a 3rd party IP submission because it's counter to my creative process. I'm not skilled enough to make something exactly like a source image. I need to be have the freedom to alter the project at any time to suit my capabilities and style or else it starts being work rather than play. Because of this, I create only original content, despite knowing that tying into a 3rd party IP may give me a better chance at reaching my goal. Good art isn't something most people can produce on command.
Perhaps what those modular designers truly enjoy is designing complex and amazing modular buildings. If doing something like that feels worth their time, the financial side is merely an added bonus.