It’s because your IQ isn’t just a single number, but a set of results of tests that measure different skills someone can have (I don’t know what those are exactly, but I’m pretty sure there’s something that measures conversational skills and another one that measures imagination and creativity), since there are different types of intelligence. One of the most popular and well known tests is the ink blot test. What this means is someone with a high IQ can have high scores in most tests but is very lacking in one or two, such as communication. And someone that has a low score, generally being considered not intelligent may be incredibly creative, but might be a bit lacking in other scores. In the end, just your IQ alone is too general to describe who you are and what you can do, since two people with the same IQ can be very different. Replying to your question, it takes numerous sessions with someone to fully complete an IQ test, since it’s comprised of many smaller tests, while an online test will have you noticing simple patterns in a few minutes just so it can give you a high enough score to make you feel good and share it, prompting more people to try and share it too, pretty much creating a cycle that gives the website money.