Why? They test in default configurations. As someone who changes many, many default settings, I don't expect developers to test in every possible combination of non-default settings, browser extensions, atypical devices, text zoom settings, etc.

These analytics scripts aren't critical and the checkout process shouldn't go down just because they don't load. It could be a user on spotty wifi or even the external service being down entirely. I agree that they shouldn't test all the functions of random third party extensions, but this failure point is pretty foreseeable for reasons besides adblock.

I'd bet there's a pretty solid correlation between using an ad blocker and propensity to spend money, if only because the good ones only run on desktop browsers. Someone who owns a keyboard is likely to be white collar if they aren't a student.

So if you want to leave money on the table because some dumb script which has no bearing on selling the product breaks, go ahead and damage that funnel.

Your website breaking while someone is actively trying to spend money on it is very very bad. It means someone who was definitely willing to pay you is not paying you. Ecommerce companies spend a lot of money on optimizations intended to keep users from abandoning the checkout process. Single-digit percentage improvements result in large increases in revenue. Ensuring the checkout actually functions is about the least a site can do to ensure users don't abandon the process.

A quick web search suggests somewhere between 25% and 45% of web users block ads in some way in 2022, depending on who, where, and how you ask.