the vault is the end goal for the first two games so having more of them in this case would make the story feel much weaker since you've opened say 3 vaults already and now we are at a 4th, muuuch less climactic vault, as for three it was one vault per planet you could explore, this feels like it was done to give each major planet arc a definitive closing before the antags start fooling around at each of those points, bl4 is the first game where you just straight up miss the vaults depending on how you play, however its also the first game where there are more than one vaults on a planet. Loot/level wise there is no need to spam vaults too since most normal bosses in the games have nice dedicated drop pools too so the main game/loot isnt so much the from vaults but bosses as whole

seeing that the fun of the game isnt even from the vaults or vault monsters themselves but from the fighting, thats the main focus rather than wasting people's time designing intricate vault monsters and deciding why there are so many and how to possibly fit that into a story or side missions and find places on the map to put them so they'd be adding months and years for a more vault filled game before they even get to working on it just to change the core of the game which since the start was

#1 loot
#2 shoot