The delay exists to prevent fraud.
So that no one sells your inventory and buys junk with the money they get. It's that simple.
But yeah, it's really annoying when you don't manage to get your funds before a game's sale ends...

Funds being pending are to protect Steam from dirty and fraudulent funds entering the Steam marketplace. Valve now waits until the monetary transaction is committed before finalizing the sale of the item.

That said, never rely on Steam Marketplace revenue to get a timed sale. Instead, use the revenue for something else later.

Because of the financial risk of fraud, this is never going to change, only potentially going to become worse over time as fraudsters seek more ways to launder and steal money unscathed.

Imagine someone buying the item with fraudulent money. The money gets withdrawn from Valve. Now Valve has to take the money away from you, but can't give you the item back because the item could have been resold by that scammer in the meantime to someone innocent who bought it with legitimate funds.

To prevent this from happening, Valve waits until the buyer's bank finalizes the transaction, making it impossible to undo before handing out the item to the buyer and the money to the seller.

This is always triggered if someone buys your item and has recently topped up their wallet balance using a new payment method.