they why we never see a 48x5 game in the NBA playoffs?
Because no team in the NBA has 5 superstars. In must win playoff game's though, it's not uncommon for superstars to play 44+ minutes. Also B3 games are played in arcade mode, where injuries and minutes don't carryover. If NBA teams could play games where they didn't have to worry about injuries or players being tired for their next game, I'm sure we'd see a lot more players getting 48 minutes.
Right now I think the situation is fine because while a 5 player lineup may be the best for the B3, there are still risks to it (like injuries and foul outs) and it's certainly not the best strategy for league and cup play. There are tradeoffs and tradeoffs should be part of BB as they make for richer strategy.
If we eliminate 5 man rosters as a viable option, then the best strategy in all situations probably becomes an 8/9 man lineup. Managers no longer have to make any tough decisions regarding roster size and makeup since there's an optimal choice that works everywhere, including league, cup and B3 play.
Not to mention, if we make it harder for player's to get 48 minutes this makes training harder especially if you're trying to remain competitive at the same time. Competitive teams may end up forgoing training altogether, leaving it to farm teams and tankers, which I think we can all agree, wouldn't be good for the game.