I do mean duration, from getting the listing to closing. It's not 9-5 work every day during that time, but there are some days that are fully dedicated to that one listing and some time spent nearly every day paying attention to it. The cognitive load of daily attention to each client and the scheduling necessary for those longer days means that an agent can only manage a handful of clients at the same time. That's the opportunity cost inherent in each client, and why agents need to earn as much as they can from each one.

The brokerage gets a big chunk because they're running an office for all of the agents: they need to rent property (usually in a prime location), build out the office space, and pay all of the costs for running that space. My wife's office has grown from 30 agents to over 100 in the past 4 years or so, and has needed to take over two neighboring spaces in the building it occupies.

Different brokerages have different policies; some charge a fixed monthly fee and take a lower cut of commissions, some charge no monthly fee and take a bigger cut of commissions. Most have a 'cap' on the brokerages cut, so that if you do well in the spring and pay out your cap, the brokerage won't take any cut from you for the rest of the year.

Taxes: yes, we all do pay taxes, but most of us only think about our after-tax net income rather than our pre-tax gross income, because the taxes come out before any money goes into our pockets. So people forget that those commissions are gross rather than net when they compare what the agent is getting to their own earnings.

I think the no-frills brokers don't exist anymore because their service was awful. "You get what you pay for" may not always work for full-rate agents because they may not be very good, but "You're not getting what you don't pay for" definitely applies to the no-frills agents.

A full-service buyers agent gets paid out of the commission that the seller pays to the listing agent. If the listing agent is a no-frills discount agent charging 2%, then the full-service agent might get 1% rather than the 3% they would otherwise get. They have fixed costs that aren't covered by that rate, and can't afford to provide their buyers with the level service they want to provide for those homes. But there's more to it than the money: shepherding a deal through from offer to closing requires a lot of work and a lot of coordination between the agents, and the discount agents just don't care to spend any time on that. It's extremely frustrating for the full-service agent to work with a discount agent who's not paying attention, and that causes a higher risk of the deal falling through or having long delays which can impact the buyer's sale of the home they're moving out of. So guiding buyers away from homes listed by discount agents can be in their best interest. (For Sale By Owner homes are also higher risk, often worse, for the same reasons.)

Realtors need licensing because there are strict laws and ethical codes that they must adhere to; there is a high risk of being sued when there is any hint of improper behavior. Agents sue each other, agents sue brokerages, brokerages sue each other, and buyers and sellers sue everyone. The licensing is there to make sure agents know what all of the rules are and the importance of following them. (By the way, insurance for all of this litigation is one of the big fixed costs for being an agent.)