Why can't powder-based 3D printing techniques create enclosed or hollow structures?
It's not hollow or enclosed structures that are a problem. It's structures that are hollow and enclosed. Think about it. The machine lays down a thin layer of powder, and then a laser fuses some of that powder together to make a shape. Then, it repeats the process for the next layer. If you try to build, for example, a hollow ball, the ball gets created in the middle of the powder, meaning that there will be powder around the outside of the ball and in the inside of the ball, and with the ball completely enclosed, there is no way for the powder that ends up inside the ball to get out. That's why hollow objects printed in this manner must have a hole somewhere to let out the powder contained in the object.

This is not only true of metal powder, but any 3D printing process that uses a bed of powder or a volume of resin and fuses some of it into a shape. Fused deposition modelling (FDM), the process used by most consumer-level 3D printers, build models in thin air, so only air is trapped inside the printed objects, making the printing of hollow, enclosed objects less of a problem. Of course, one has to deal with gravity. A hollow object might collapse before the filament hardens enough to support its weight.