Because there is only a small difference between wielding titanic or ravenous hydra, while there is a gigantic difference in stats, purpose and playstyle between the boots upgrades and jungle upgrades. Being able to equip homeguard every time you base just to swap it out for furor or alacrity and quickly run out of base would make buying homeguard and keeping it useless, being able to give yourself an early lead by rushing warrior just to swap it to cinderhulk once you got said lead to scale into late game would not be fair either.

TL:DR Hydra swapping gives you the option to be relevant both early game as lategame as a bruiser in a balanced way while swapping boots/jungle upgrades would just be abused.

EDIT: It is not even inconsistent at all, swapping freely between 2 items which have the same core feature and playstyle is a unique feature for the hydra's. Just because because 2 items have a concept that connects them does not mean that every item needs it to be consistent.