The reason is quite simple: unless you work in the hospitality industry, you are better — able to extract from it a higher income — at doing whatever else you're doing than you are at being a host, or you wouldn't take it up as a gig, but rather switch to it full time. Suppliers in the gig economy (as opposed to professionals freelancing in their area of expertise) are by definition working more hours but less efficiently so, whether because they don't have the training and experience, or because they aren't working with the tools and resources they'd take advantage of in their regular environments. The cheaper, less quality, badly regulated service they provide might be desirable to many customers, but this is achieved partly through de-capitalization. Every hour and dollar an accountant spends caring for a guest instead of, if he wants a higher income, doing more accounting or upgrading his tools, is a waste of his knowledge. From the point of view of overall capital and skill intensity, a professional low-budget hotel chain would be vastly more efficient over the long term (of course, to do that you need to invest capital in premises and so on instead of on vastly cheaper software and marketing).

The only reason for an accountant, web designer, teacher, or what not, for doing "gigs" instead of extra hours, freelance work, or similar, is if there is no demand for their professional labor. While it's entirely possible that overtime or freelance work might be relatively less valuable than the equivalent time spent at their main job, to do something else they would have to get less than what they can get from a gig for which they have little training and few tools. That's not how a capital- and skill-intensive economy looks like.

For an specific occupation falling out of favor, this is just the way of things. For wide swaths of the population to find themselves in this position, perhaps employed but earning less than they would like, and unable to trade more of their specialized labor for income, the economy as a whole has to be suffering from depressed demand. What's more, they still have to contend with competitors with more capital but still looking to avoid regulations (e.g., people buying apartments specifically to rent via Airbnb), in turn lowering their already low gig income.

This is a good thing if you want cheaper human-intensive services or have invested on Airbnb and similar companies, and it's bad news if you want an skill-intensive economy with proportionally healthy incomes.

In the context of the gig economy, flexibility is an euphemism for I have a (perhaps permanent!) emergency and can't get extra work, and efficiency refers to the liquidity of services, not the outcome of high capital intensity. And while renting a room or being an Uber driver might be less unpleasant than, and downright utopian compared to, the alternatives open to those without a room to rent or an adequate car, the argument that it's fun doesn't survive the fact that nobody has ever been paid to go and crash on other people's couches.

Neither Airbnb nor Uber are harmful on themselves — who doesn't think cab services could use more a transparent and effective dispatch system? — but customer ratings don't replace training, certification, and other forms of capital investment. Shiny apps and cool algorithms aside, a growing gig economy is a symptom of an at least partially de-skilling one.