Because they don't know anything about the trading card industry.
A 2024 Donruss Green Pandora Optic Preview 379 Drake Maye [RC] that's professionally graded a "10" (serial numbered to 25) has sold for only $205 on a live stream.
The seller canceled this order, claiming that a glitch prevented people from bidding higher. It didn't sell for higher because bids were limited to 20 seconds. They previously had this card listed for $1,375.00, claiming this is an incredible offer because it's worth over $2,000.
Claiming this card is worth over $2,000 is a stretch because only one copy has ever sold for this amount of money. The other copies sold for anywhere between $730 to $1,040.
This is the same seller who allegedly agreed to cancel an order for a counterfeit trading card on another live stream, only to deny the request off-stream. The buyer claimed this card failed the authentication process, and they got their money back.
There's a group of people who predicted that live streaming is the future of selling high-end trading cards. I'm not entirely convinced this will be the case if sellers are allowed to cancel bids they don't agree with.
It's sellers like this who are making the secondary market volatile. It has reached the point that cancellations are a common practice among sellers in this group.