Because they’re meant to offer a single-package solution, as we said above. The whole point of portable is simplicity. If in addition to the projector you also need to drag along external speakers, things stop being convenient real quick. Thus, good portable projectors have built-in speakers that sound good even in the great outdoors. We’re not talking stadium rock caliber, but certainly good enough for a campsite or something like that.
Simplicity is the rule with portable projectors, whether you’re on a road trip, at an impromptu business meeting, or having a birthday party for the kids. You don’t want to mess around with extra cables and spend time setting up speakers, not if the integrated audio is good to the point of making all of that completely redundant. And related to simplicity is space efficiency. If you’re in a small room like a dorm, then having more isn’t always a good thing. Great video and audio from one device beats the clutter of video and audio from multiple separate devices, each requiring its own power adapter, placement space, and so on.
Even better, if you look at high quality portable projectors, they have Bluetooth and by that can function in speaker mode for other devices, like your smartphone. That’s a nice extra, because portable projector speakers offer a lot more power than the audio output provided by smaller devices like phones and tablets.