A few hypotheses:

1. Other people enjoy fun-looking spaces much less than I do
2. It’s more expensive to make a place look fun so it’s not worth the extra cost
3. Fun designs are appealing at first but people who look at the same “fun” stuff daily eventually get tired of it and would overall prefer a plainer option

My number one hypothesis here would be that fun spaces are generally not considered high status. I think maybe the association with children/childishness could drive this, and reminds me of this post, with a guess for why velcro never caught on:

cecie:  Consider a simpler example: Velcro is a system for fastening shoes that is, for at least some people and circumstances, better than shoelaces. It’s easier to adjust three separate Velcro straps then it is to keep your shoelaces perfectly adjusted at all loops, it’s faster to do and undo, et cetera, and not everyone is running at high speeds that call for perfectly adjusted running shoes. But when Velcro was introduced, the earliest people to adopt Velcro were those who had the most trouble tying their shoelaces—very young children and the elderly. So Velcro became associated with kids and old people, and thus unforgivably unfashionable, regardless of whether it would have been better than shoelaces in some adult applications as well.