Wooden furniture is not treated. You are thinking of pressure-treated lumber, which is not used for furniture, but rather for outdoor wood in contact with soil.

Staining is a treatment. So is waxing, or varnishing.

I’m sure some people leave their wood furniture unfinished but it’s certainly not the standard, and not how you make them last.

Completely untreated wood stored in a dry environment will last many years (hundreds? tens at least) without degrading. Look at woodworkers hoarding wood in basements, beams in old buildings, etc

Staining/waxing/varnishing is used to protect the surface from wear and tear or change aesthetics. A varnished table that has been worn through in areas can be sanded back and look like new. The wood under worn spots won't be noticeably different.

I've never heard someone in woodworking call finished wood "treated". That would be really odd, given that treated wood is a specific term of art.

Plainly oiled wood is pretty normal, and isn't going to do anything to protect against rot (because it doesn't need to indoors).