This sad fact is because most pet door flaps in use today consist of a sheet of vinyl fastened rigidly across the top of the frame with screws. As the pet goes through the flap it flexes. Worse, the pet must press upward against that flexed area to get full use of the space. 

Eventually, the repeated flexing causes the vinyl to crack and tear in just the same way that repeated flexing of a coat hanger will cause it to break.

But it doesn't flex.

And that simple design difference allows the Endura flap to be flexible and extremely durable at the same time.

The Endura flap is warranted against failure, except for chewing damage, for 15 years.

By the way, since this illustration was designed, the test machine went over 3 million trials. Same result.