Why does the seller pay the commission for both agents?

This is actually changing. The NAR settlement in 2024 removed the requirement that sellers offer compensation to the buyer's agent through the MLS. So the structure you're questioning is no longer the default in most markets. Historically, the seller paid a total commission that was split between the listing agent and the buyer's agent. This became the norm decades ago because it was simpler, it ensured buyer's agents were compensated for bringing buyers to the table, and it allowed buyers to get representation without having to come up with additional cash on top of their down payment and closing costs. Now, sellers can choose whether to offer buyer agent compensation at all, and if so, how much. Buyers are increasingly being asked to negotiate their own agent's fee, either paying it directly or requesting the seller cover it as part of the offer terms. In practice, the market is still adjusting. Many sellers continue to offer buyer agent compensation because it attracts more showings and keeps the buyer pool as large as possible. Others are offering reduced amounts or nothing at all. There's no single right answer, it depends on your market conditions and your goals. The bottom line is that you now have more control over what you pay as a seller than ever before. Discuss the options with your listing agent and make an informed decision based on your specific situation.

Commission is completely negotiable. When listing your home with an agent you will want to discuss this with. If you so decide to not offer out a commission to a buyers agent there are some concerns with. If a buyers agent shows your home and there is no commission offered, then they will ask the buyer to pay for the commission. If the buyer doesnt have it, they cant make an offer on the home. If they do have it and other sellers are offering to pay the buyers agent then the buyer interested in your home will often offer less for the home to compensate for this charge that they will have to pay. As a sell you dont have to pay for anything or accept any offers you dont want to, though just consider why they may be in place as a benefit for both buyer and seller.

This is a great question. Everything in real estate is negotiable.