Have you tested that? Have you for example given a tarantula a choice between different burrows surrounded by different light levels and amounts of cover? Have you tested out what a tiny heat gradiënt does? And that for different spiders. This seems just as how people justify keeping snakes in tiny cages or monitors in exhibits they can barely turn in.For most animals, yeah I'd agree, but a Tarantula doesn't need and wouldn't even use multiple choices or a large environment. They prefer a small space, so a large enclosure just makes them hard to see and may actually be detrimental to them.
If we follow that kind of logic, that would also mean that a tarantula in the wild would just choose the first possible option to have a burrow. Regardless of wether that spot is let's say wetter/drier, darker/sunnier, covered/open,... You're suggesting it doesn't take it's surroundings into account to choose it's burrow wisely. Whilst they will probably only use a fraction of the exhibit the vast majority of the time or even all the time, this doesn't mean that if you release them they won't be taking the whole exhibit into account when choosing where to live.