the reason for this restriction is simply that a base becomes virtually untouchable if you could raise ground infinitly or lower it infinitly. and the water level thing is probaly beyond the devs possible changes simply as it would require them reworking the entire water system of the whole game. that is too much effort for too little gain overall and would take way too much devtime away from actually adding content.

these limits simply exist for balance reasons. and i doubt theyll ever change them much. they might raise the limits from lets say 8 to 10 or 12m but definitly never infinite. thats asking to break the game completly.

picking a good spot for a base is part of the experience aswell. if you could alter terrian infinitly anywhere this pretty much is removed from the game.

plus another reason is probaly the games performance. in the earlier versions exessive terrian changes lead to MASSIVE fps drops when loaded in. without a limit some of these issues might resurface again aswell. its alot better now but you can still make the game lag like crazy if you bypass that limit with cheats or mods and use it excessively.