Why does a raft adhere to the bed while skirt does not?
The raft base (initial layer) is usually printed with very wide lines. Cura's default is double the nozzle diameter, so 0.8 mm with standard 0.4 mm nozzle. This is ridiculously high flow, especially with the default 0.3 mm thickness, which is why the raft base lines are (and must be, unless you have a seriously overpowered hotend) printed so slowly. So, each line has significantly increased (double) surface contact, more pressure against the bed, more forgiveness if the bed clearance is too high (since it will still smash against the bed, just spreading out less than the whole 0.8 mm width), comes out slower (giving it better opportunity to adhere), and does not involve any curves (where the nozzle may &quot;pull&quot; just-extruded material that's not yet adhering).
You may be able to get a lot of the same benefit with skirts (or brims) by increasing the &quot;Skirt/Brim Line Width&quot; setting and slowing down its speed.