Bidiots - why does artist not get money purchasing your own painting

The artist has too much information

If their low value painting ($500 - $2,500) has sold then they know that their other painting is high value ($3,000 - $5,000), or vice versa. This bypasses the intended mechanic of guessing the painting price based on the name and other people's bids.

To add risk to self-bidding

The average painting price is $2750. The artist would get:
• $(bid/2) if someone else buys their painting.
• $(2750 - bid) if they buy their own painting under current rules.
• $(2750 - bid/2) if they buy their own painting under suggested rules.

Under the current rules, the artist can achieve greatest average profit by self-bidding up to at most $1800 (the last value until bid/2 exceeds 2750 - bid). Additionally, they can bid up to $2700 before making an average loss.

Under the suggested rules, the artist could achieve greatest average profit by self-bidding up to at most $2700 (the last value until bid/2 exceeds 2750 - bid/2). Additionally, they could bid up to $5500 before making an average loss.

(Though bidding up to $1800/$2700 isn't necessarily the best strategy if you think you can make a greater profit by using the money elsewhere, because of how buying a painting locks up that money until the end of the game.)

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So, while I can't speak for developer intentions, the artist likely loses the money to prevent the meta being to only bid on your own paintings. Instead, self-bidding is a risky calculated move, usually for situations where you're relying on someone else out-bidding you.