In order for me to pay you, I at a minimum have to do some amount of mental gymnastics to convince myself that it's worth it for me to pay you. This has a perceived cost even if the money spent is trivial. This is why people who take money in small increments - i.e. mobile games, arcade operators, casinos, and so on[0] have you buy a large amount of some scrip that they control, and then make it so easy to spend it that you might accidentally do so.
Nobody is thinking "I'd buy this, but only if I can leave no record of ownership[1]", they're thinking, "is it actually worth buying". Identity and privacy isn't a thing that people actually account for when making purchases - mostly because it's never actually mentioned[2] in the terms of purchase. It's snuck in. So the choice is just "the free one" and "the $2 one", where the value of the $2 extension can never hope to overcome the mental transaction costs.
[0] Nintendo and Microsoft used to do this around the Wii and 360 eras. While on the Wii it was 1 point equals 1 penny/yen, Xbox did something nasty and made it 80 points equals 1 dollar.
[1] That would mean that setting up a new computer or browser profile loses you all your existing extensions that you paid for.
[2] I do not consider legal disclaimers to be adequate notice, and neither should you. Dropping a clause in a EULA is the equivalent of dropping rohypnol in your drink.
And also unlike cash a service can keep billing you.
And, for better or worse, the risk is partially put on the business in the form of increased cost (or payment service denial) when a credit card transaction is considered too risky (charge backs).
Also there is a fair amount of friction to giving payment info than say pulling out your wallet or phone (but this is improving with “digital wallets”).