Betfair takes its commission as a percentage of the profit that one has made in a given market rather as a percentage of a bet size, so it's always worth making a bet on a certain event. Usually by the end of an in-play sporting event, one will find that there is no liquidity available to bet for the likely winner or against the likely loser (and draw if applicable) at any price (their lowest price is 1.01 BTW).

In this case, there are clearly people who are still willing to bet against (lay) a Biden Presidency, at very long odds. This is presumably either because they believe that the price adequately compensates for the low probability of it not happening, or to hedge risk that they have accumulated taking other positions while trading in this market (more likely IMHO).

One other factor is the cost of carry - while money is tied up in a bet it can't be used for anything else, including other betting strategies that might have a higher expected profit. In this case, if one has already made a profit betting for a Biden Presidency at longer odds than 1.07, it might now be worthwhile to pay someone else the slight risk premium to take up ones position until settlement, so that one can lock in the profit and move onto other things.