I just encountered the following error:
Oops! Your question couldn't be submitted because:
users with less than 125 reputation can only post questions every 20 minutes; try again later.
Why do you have this rule? Why 20 minutes?
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To avoid spamming.
125 rep is not that hard to get.
Both intentional and unintentional (you would not believe how much time you can spend explaining to new users that they should edit their questions not re-post them every time they want to make a itsy-bitsey little change...).
In addition to stop spammers, it is also a way to make you write better questions.
If you're churning out more than 1 question every 20 minutes, you're not putting much effort into the questions.
The more effort you put into a question, the better answers you get, and often effort equates to time spent composing it.
In Stack Exchange there are at least millions of users online that are not question banned (if I am not wrong) where each user can post new question.
Of course not necessarily and not probably that all the users are posting their question at once but it might happen in a difference of few seconds and the servers will have to handle about few hundreds of thousands of posts, which is proven that the servers can handle.
If each user is unlimited in posting new questions in time then think what would happen if all the millions of users online are posting new questions constantly and non-stop this will create a huge pressure on the servers and might even cause the servers to crash and shut down.
The only reason that this is not happening is exactly because of that question limit that you are complaining about in your question.
Few hundreds of thousands of new posts per 20 minutes is not enough big pressure on the servers and it is already proven that the servers can handle this.