Because it’s a game of chicken between the landlord and the tenant. The landlord usually has some rules for notification (e.g., 30-60 days before expiration) and the tenant usually has something similar for notice to leave.

Nothing stops either from notifying or agreeing to a longer agreement before the old one expires. Though with larger buildings (vs individual owners) it’s less likely they’d be willing to diverge from their standard operating procedures and timelines.

Define "in general". All big landlords (Greystar and such) want you to sign lease extension 3-4 months before the current term ends where such thing is legal. There are states that don't allow long term leases past the first term though. For example, in California all leases go month-to-month after the first lease term but they put required notice periods on rent increases (30 days and 90 days if more than 10%).

The exception is if you are in a college town, when many apartments/houses are pre-leased starting before spring break for the fall. I see that all the time in my current college town.