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.

Small electrolytic capacitors (diameter 5 and 6.3 mm) usually don't have vents because:

• It's hard to manufacture a reliable vent on such small surface
• They don't generate enough energy (pressure) to be dangerous
• The bottom sealing is weak enough to blow relatively early in case of failure

As said before many axials, old or cheap electrolytics don't have vents.

Big screw terminals electrolytics have vents, it is generally a small hole with a rubber plug, not a weak point engraved in the housing.

You should not underestimate the risk of a capacitor blowing. I've seen medium sized ones (diam 16-18 mm) make a dent in a steel 2 mm chassis, the same dent you can make by hitting it very hard with a hammer. Even if there is no fire or projection, the noise of a big capacitor blowing can damage your hearing if you are too close.