The same could be asked of many supplies like baseballs.
My guess is these supplies are ordered in great quantity for the season or a large part of the season. While carrying large inventory can be expensive, maybe clubs have some sort of financial advantage doing things this way. Or maybe no one is paying attention? I doubt that.
"Hating the Yankees like it's a religion since 94'" RIP Mike.
"It's also a simple and indisputable fact that WAR isn't the be-all end-all in valuations, especially in real life. Wanna know why? Because an ace in run-prevention for 120 innings means more often than not, a sub-standard pitcher covering for the rest of the IP that pitcher fails to provide. You can't see value in a vacuum when a player does not provide full-time production."
My guess is that they typically warehouse everything close to Boston. Supplies from Spring Training would have to be sent back to Boston at some point before the start of the season anyway via truck (all the MLB player helmets, bats, etc.). May as well just ship everything to Boston and stock the truck there.