Around here to run a taxi you need several things. One is a commercial vehicle driver's license, which requires training above and beyond the normal license. You also need commercial insurance, to protect yourself, your passengers, and the insurance company (else they would have to raise premiums for everyone to accommodate risky, illicit behaviour). Taxis are required to have cameras, to protect the drivers and passengers. From what I've seen of UberX's 'requirements', they violate in nearly every way, and I don't think I need a 'source' to state why a lack of training, insurance, and safety measures could harm people.
As for AirBnB, imagine you own a condo and someone across from you is running an illicit hotel. These kinds of things affect safety (random people who don't live there constantly coming and going), property value, and hurts legitimate businesses that actually employ people (say, the real BnB or Hotel nearby that follows all the regulations and thus has to pay higher costs). And of course, if you own a condo, are renting is, and your tenants are illegally subletting, the damage is more direct.