Mostly because the tradeoffs are poor for a built-in flash. A tilting pop-up unit would need a more complex hinge/mechanism, making the camera larger, more fragile, and more expensive. Bounce flash also wastes a lot of light, so the tiny built-in flash would need more power to be useful, which is difficult with limited size and the camera’s main battery.

In practice, built-in flashes are already weak, so a tilting version still wouldn’t approach the flexibility or effectiveness of an external flash. That makes the audience for such a feature fairly small: people who want better flash than a fixed pop-up provides, but not enough to buy a hot-shoe flash.

There have been a few exceptions, but the idea hasn’t become common. Camera makers generally judge that the added cost, bulk, and reliability concerns aren’t worth it, especially since photographers who care about bounce flash are often better served by an accessory flash.