Sometimes the merit is one of the main reason why other submit their arts since there are generous forum member gives a good amount of merits on the submissions made by the participants so even if they didn't win in the contest still their effort has been paid off due to the merit they get. Maybe some other didn't expect with the prize since there are so many talented forum users join the contest.
I am not agree with your point that you forum member submit their arts to get merit. They submit their art as it is a competition. It is not a fact either become winner or not. If those person who would not be winner would not take part on the competition then no competition would happen in the world.
Roobet host a lot of great art competition and that is not about merit though, the reward is big so if you have the talent to art why not participate?
I also don’t think people participate for merit because as I can see on many art contest with Roobet not all arts have merit, meaning there’s no assurance that you can get merit and if I were to participate, I will surely aim for the reward and not for merit.