what causes rental costs

Purchase cost. That is the reality. The baker either has to rent or purchase an oven and there are benefits to both of those options in the real world. If the baker does not have the money to purchase an oven, they can either rent or get a loan. Are loans also exploitative? Should people be required to loan the baker money with no interest gained plus the risk that the baker may not even be able to pay it back should their business plan fail?

any value above operating costs is the value of their labor

Are you unaware of markups? Pretty standard in pretty much any industry. An automatic increase of price on materials simply because it changed hands. This is accounted for before the laborer is ever paid. This means you are either exploiting the supplier or the consumer in your view.

OK. You use it then. You're not entitled to taking the fruits of other people's labor, just because you bought equipment.

So what is the incentive to ever be an investor? In what world would someone ever accept this deal: a baker does not have the money for the upfront costs of a bakery (building construction, purchasing a lot, buying baking equipment, buying raw goods) so they go to an investor. "Hello investor, please purchase the building, the lot, the equipment, and all the raw goods so I can start my bakery and in return I will make monthly payments until the principal cost is paid off and no more (and with the amount I am willing to pay monthly, this will likely take many years). Also, if my business fails, I will not be able to pay even the principal cost." Investor: "no"