The highest offer may not be the best one. Your agent is not necessarily about making more money, they are about determining which offers will actually close. A great offer with a condition or from an unvetted buyer may fall apart and you are back to square one.
100% of the time, clients identify "honesty" as the most important qualify in an agent. Here is the rub, sometimes your agent may tell you something that may be uncomfortable to hear. An agent who only tells you what you want to hear is not doing their job.
A part-time agent is simply not competent to represent you. Deep market knowledge, networks with other agents and which negotiation levers work/don't work are a full time career.
You want legal advice, you pay a lawyer an hourly rate. You want to speak to your lender? Those calls only happen between 9-5pm. Chances are your realtor is available almost always, at no charge to you until you close.
They do not get paid until deals close, and that could take years. That is years of calls, emails, site visits and consultations with third parties without a paycheque.
The barrier of entry is low, but the ability to maintain credibility (both among clients and within the industry)... THAT is much much higher.
You may not want to deceive someone, but you may still be negligent if you misrepresent the condition of your home. A good real estate agent can keep you out of a court proceeding.
A word of caution about homes being sold by owners: someone selling their home themselves does not have to follow the same ethical rules real estate agents do. If you find the home you bought has a leaky basement, you may have no recourse. An agent can help you navigate these risks.