Uneven paint on a smooth and primed wall
Paint will not fill holes, voids, etc very well.  It CAN do it, but it takes many coats and will still never look right.

If you want a smooth surface when you paint, you need to start with a smooth surface.

For a wall like this, go over the whole wall with medium grit (120ish) sand paper to knock down any high spots and roughen up the surface a bit.

Then, using a very wide blade trowel, apply a very thin "skim coat" of joint compound (light duty will be fine) to the entire wall.  You're not looking to build up the wall - instead you're looking to fill in all the voids without adding to the overall thickness.  Hold the blade of the scraper at a 45° angle, apply the compound evenly but with a fair amount of pressure where the edge of the blade meets the wall to keep it very thin.

Once the compound is mostly dried (not soft to the touch but not bone dry), take a SLIGHTLY DAMP, LARGE (big as you can find) sponge and go over the wall lightly to -- well think of it kind of like buffing the wall.  Only don't do lots of circles...  just go over the wall to take down any ridges/lines left behind and get that nice smooth surface.

Finally PRIME the wall, then paint.