Man, but we invented the calculator and the computer for that!

Yes we did, but there’s a difference between delegating a task (asking a computer to do it) and executing the task (running the calculation). Otherwise you might as well say humans can run 40mph because we can ride horses.

Also, no one person invented the calculator. The calculator is the culmination of hundreds or even thousands of years of invention. It’s not like the knowledge or creativity is in each of our brains and we could each build a calculator given the requisite materials. It took thousands of lifetimes of ingenuity. So there’s another answer to your question of things we aren’t efficient at solving: building a calculator.