And without the investment angle, there's no bubble.

A bubble isn't "they're really popular right now, and there's a whole bunch of them popping up". That's a fad. A trend.

A bubble is always in reference to investments. It's a pump-and-dump scheme at the level of the whole economy. The housing bubble isn't because there's a glut of houses on the market, it's because people are trying to market houses as an investment opportunity for market squatters and landlords.