Our "homes," whatever shape those homes might take (apartments, cottages, dorm rooms, the van in which we full-time travel, etc), tend to relax us.
That relaxation isn't a given, especially if there's some other factor we associate with home that pushes us in the opposite psychological direction (a leak in the ceiling, an abusive partner, a baby that stresses us out and keeps us awake), but in general our homes are physical spaces that feel predictable, secure, and us-shaped.
Part of why our home spaces nudge us in these (generally positively valenced) directions is that we know our homes, they’re familiar. Folks who have just moved into a new apartment won't feel that same sense of ease and comfort in their new space until they've lived there a little while, and folks who travel a lot for business, spending a little bit of time in dozens of hotel rooms a year won't necessarily establish the same sense of place, either.
This may be why some research has shown that clutter in one's space can diminish the sense of comfort one feels in that space, and can even make them feel psychologically displaced and alienated from not just the geographic confines of their living area, but from a sense of "home" as a theoretical concept.
Other research in this space has shown that clutter can lead to more unhealthy snacking, suggesting that we might reach for soothing, if (health-wise) non-ideal comforts when we feel less in control of our environments.
This lines up with other research that shows we tend to look for quick-hits of pleasure chemicals when we feel stressed, socially isolated, or rudderless—things that seem to be associated with a cluttered environment.
These effects would seem to be even more monumental for folks who are real-deal hoarders, but most of the research focuses on non-hoarders who just happen to have messy homes, usually because they have too much stuff in them for the space they occupy, or are too busy or stressed-out with other things to bother putting stuff away on a regular basis.
We don't know what causes this seeming association between these sorts of environments and this type of response, but it's been posited that a cluttered space has so much going on, and so much of all that stuff is ever-shifting, that it's impossible to establish a concrete mental map of the space—which is what would usually allow the person living in that space to slowly build up a heuristic for it, helping their brains relax into thoughtless, energy-sipping (rather than energy-guzzling) routines when home amongst all that familiarity.
It's also been posited that clutter poses a near-constant distraction, keeping the portions of our brains that look for threats and opportunities occupied all day, which in turn disallows full relaxation and uses up a lot more energy than an uncluttered, predictable home would necessitate.
We don't know for sure this is why research almost always seems to point at clutter as a source of stress, and we don't know for sure that it's exclusively stress (and not some other aspect of a cluttered lifestyle, or the mindset or habits that lead to clutter) that tend to lead to the recorded consequences.
It’s also worth considering that clutter has (at times) been associated with increased levels of creativity—so it may be that to feel healthy and secure we should keep things tidy, but we might allow a little mess to accumulate (at least for a while) to periodically stoke our creative side.