It can seem that way, but the consequence of not knowing them is lots of subtle race conditions. Granted, most webapps probably don't have enough users to trigger them too often, and the impact is often limited e.g. because the kinds of inconsistencies you can introduce don't actually matter that much in many cases.

For the majority of webapps, I'm not sure the default isolation level of MySQL and Postgres is an appropriate tradeoff, since I'd rather have correctness by default than performance by default.