Why does a SYN or FIN bit in a TCP segment consume a byte in the sequence number space?

It’s not particularly subtle – it’s so that the SYN and FIN bits themselves can be acknowledged (and therefore re-sent if they’re lost).

For example, if the connection is closed without sending any more data, then if the FIN did not consume a sequence number the closing end couldn’t tell the difference between an ACK for the FIN, and an ACK for the data that was sent prior to the FIN.