Because accurately estimating the time needed to solve a problem is usually at least as difficult as solving the problem itself.

Here’s a thought experiment for you: How long would it take you to solve a crossword?

You don’t know the solution, and honestly, you don’t know how long it will take you to find the solution. If you’ve done a bunch of crosswords before, you might have an idea, but some might have been done in an hour, some might hace taken all day, some you might come back to after a few days when you have inspiration. And until you actually do it, you honestly have no idea.

Finding and fixing bugs is really not so different.

Because accurately estimating the time needed to solve a problem is usually at least as difficult as solving the problem itself.

Here’s a thought experiment for you: How long would it take you to solve a crossword?

You don’t know the solution, and honestly, you don’t know how long it will take you to find the solution. If you’ve done a bunch of crosswords before, you might have an idea, but some might have been done in an hour, some might have taken all day, some you might come back to after a few days when you have inspiration. And until you actually do it, you honestly have no idea.

Finding and fixing bugs is really not so different.