Why do some capacitors not have vents?
Do you mean the "T", "X" or "K" on top of most electrolytic capacitors ?



Actually these vents are not vents but a deliberately made weak-point in the housing of the capacitor.

The vents are only needed for Capacitors which contain some electrolytic fluid which could start to boil and create pressure.

Not all capacitors contain electrolytic fluid, for example "Solid electrolytic capacitors" or "Polymer capacitors" don't.