eli5: Why do bugs in software exist?
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Because people are not perfect neither are they omniscient. Programmers have limited time and resources to get stuff done.
It’s important to understand what Bug is. It’s unexpected behavior.
In a complex system it’s inevitable that from time to time some parts will interact in some strange nonstandard way.
Another reason can be unexpected input. Some users are idiots and no matter how idiot prove you make your system there will always be better idiot who finds a way to break it.
Programs are made on assumptions. You need to make great many predictions about how your system will be used long in advance and this leads to situations where your assumptions were wrong.
When for whatever reason your assumptions are no longer valid you need to step in and make changes in the system to make it work with new assumptions. But you are rarely if ever allowed to rework this from ground up. You just tear the wrong parts out and duck tape your changed functionality in.
And these assumptions can and do change all the time. So you will end up with duck taped mess holding together by the tireless effort of countless programmers constantly fixing them.
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