but I know working for Uber is actually extremely tough and doesn't pay well at all

It doesn't pay well, but there are incomparably more drivers than there ever were Taxicab drivers. It is supplemental income for most of drivers.

But all that aside, it's interesting that so many large companies now don't actually own the assets they have their employees use for profit-generating. They lease it.

More often they act as intermediaries to facilitate business, handling administration and adherence to the law within the parameters of the industries they are innovating. As far as leasing goes, that's not a widespread development in these industries, although that is the path Uber and Lyft specifically plan on going down.

The instability is probably the most worrying part of leasing everything rather than owning.

A lack of assets isn't stopping these companies from operating for years, losing billions every year. Their investors view these losses as investments in growing market share and innovating cost reducing implements.

If a large number of companies use this "leasing assets" strategy, then the whole economy could fall over the first quarter all these companies have a bad quarter and are forced to fold because they can't make the lease payments and are maxed out on debt.

This is unlikely to happen. Uber for example has raised something like $100BN+. They could continue losing money for another 20 years before burning through that. The investors in these corporations are playing the long game. They expect that Uber will one day be a trillion dollar corporation.