I certainly wouldn't expect it early game- but by the time you have the territory and resources to waste a few tiles, it makes total sense to me that your capital world would become the heart of your civilization's culture. Bear in mind, tombs and gardens and such are something tied to autocratic governments- the sort liable to invest in vast vanity projects and shows of their might.
I doubt you can just infinitely spam unity statues- there's probably a small set of unity-boosting structures with different in-universe purposes.
Indeed, it makes sense in the context of an autocratic regime showing off. The culprit here might be the fact that it's unclear what these monuments actually produce. Someone here likened the resource to government bureaucracy, whereas the impression I'm getting from the description is more something akin to cultural developement. Also the other buildings have some realistically quantifiable product and monuments up until this point have worked to enhance the buildings and populace involved in some aspect of production or developement. But if the monument itself produces something somehow (I'm assuming they'll still require population for maintenance) then from a gameplay point of view it becomes viable to create specialized monument planets (if for instance we get a building or edict that enhances the planetary output of all monuments) a strategy that would make sense in game but realistically cultural developement can't be mandated in this sense in a top-down fashion of "we'll build all our culturally significant landmarks here" when those arise in a more periodic and spontaneous manner.
To sum up: if this system is to represent a kind of cultural developement, it would make more sense to encourage to disseminate monuments rather than focus them to at least simulate that the different planets have a bit of their own culture to them. If it's a system of governance then yes, a government hub is pretty sci-fi like and I don't see a huge problem with it (but seriously, government control monuments covering a whole planet as efficient instracture?)