1. They are air-tight, the plastic can deforms with temperature changes and can split, causing all the gas to leak out. I replaced my last one after two years because it was split partially and looked likely to fail soon.
2. The can builds up pressure in warm/hot weather that is hard to release without gas and fumes spewing out too fast from pressure inside.
yep, not air vent. The old ones had that with a removable “snap” to cover/uncover