Two reasons.

First, phone cameras all have a fixed aperture. If you set the aperture wide enough to get bokeh in a typical portrait shot, then the depth of field would be too narrow to be practical for general photography.

Second, the depth of field depends not only on the f-stop but also on the focal length (or, to put it another way, it depends on the absolute diameter of the aperture). Given the focal length of a typical phone camera, you'd need a lens with a very wide aperture to get significant background blur for subjects at portrait distance. Wider apertures complicate the complexity and expense of lens manufacture.

It is interesting to speculate on the use of very wide aperture lenses on future phone cameras, though. If such lenses could be made cheaply, one can imagine that the shallow DOF could be overcome via some form of automated focus stacking.

Totally not an expert but isn't that because the lens itself is so small (w.r.t. distance to sensor) that it effectively is a small aperture and thus incapable of generating a narrow depth of focus?

Also, you probably want the depth of focus to be large for general purpose photography. And you don't have a variable aperture in that phone (I guess).

Because of a few things : 1) in a camera, you’ll have a big (>1inch) sensor. 2) in a camera, you’ll have a much bigger lens diameter, thus a bigger aperture diameter (not f number). Both effects will decrease the width of the focus point, wich will increase the bokeh effect.

Because phone camera sensors are so tiny, and the physical focal length so short, the perceived depth of field is enormous; basically infinity all the time. The aperture that would be required to achieve physical bokeh at normal portrait ranges would be impractically huge.

Because you are limited by the mobile form factor. The camera lenses which produce these kinds of pictures can weigh 2lb alone. The sensors are also huge — over 800 sq mm. Without the hardware you can only match the DSLR optics by using software techniques.

Short answer, you are geometrically limited by the phone dimensions. Slightly longer answer: similar geometry and cost constraints imply a fixed aperture lens, so you have to pick one that works for lots of situations.