Why must living things rely on thousands of catalysts for chemical reactions necessary for life?

Well, you don't need enzymes (biological catalysts) if you're willing to wait a century or two to digest a burger.

Without catalysts, complex reactions like digestion would take too long and the organism could not extract energy from the nutrients it eats in a practical time frame.

In addition, speed is everything in the biological world.

Some reactions and their speed relative to other organisms reactions determines who survives and who doesn't, among other aspects of life.

If a plant is slow to photosynthesize and grow in a habitat high in competition for sunlight real estate, other autotrophs will surely take over.