I was thinking more along the lines of what
@Joey said: there isn't enough time to get everything done in one day. So, instead of expanding the offering like European parks, they artificially restrict the offering so you have to go for multiple days if you want to get everything done.
But, as you've pointed out, there's virtually nothing for you to do after the park closes.
- The treetop course has been mothballed after just 2 seasons.
- The waterpark is open for one hour after park close, barely long enough to get back, get changed and get in before you have to get back out again (assuming you didn't dare to get in a queue that takes longer than park close).
- The spa is the same (and, apparently, a massive disappointment now).
- To be fair, the golf is open until 8:30pm, but that's basically a one and done thing
So after 7pm, you've got diddly squat open that isn't the "arcade" in splash (assuming splash is open that day), or the band on the pirate ship. Compared to Phantasialand, where the Ling Bao pool is open until 10pm (no slides but included in the rate), along with the spa as well.