Because although you shouldn't do it for privacy reasons, there is nothing at a protocol level to stop you from doing it. Lots of people reuse addresses many times, and so will end up in with multiple inputs on the same address. In some specific circumstances you can reuse addresses without losing too much privacy, for example, if you are receiving multiple payments from the same person over a period of time. In such a case you still reveal the total amount you have received, but you at least avoid linking it to payments received from other sources. Lots of big services and exchanges reuse the same hot wallet addresses thousands or even millions of times.

Accidentally, inexperience, using a bad wallet that doesn't generate new addresses, doing so out of convenience while accepting the risks involved, etc. There are lots of reasons. It is far higher than 1% of addresses which are reused.