To iterate fast and get something in the hands of users. I can guarantee they want their app on every platform under the sun someday, but things take time.
But I'm sure this is restricting the number of users who access it as "people who use discord" is a significantly smaller number than "people who use the web".
Discord solves a big problem for them: they are trying to give a limited number of uses to each user. They dont have to create their own proof of human infrastructure since discord already has some weak forms of that with sms verification.