If you’ve ever turned over an engine by hand you know it’s no easy thing to do. You’re working against a lot of internal friction, plus the compression as each piston rises on the firing stroke. Your starter has to do the same job, except a lot faster. So it clearly needs to be built well.
Take a look at these two starters for a Land Cruiser F or 2F engine—an aftermarket unit on the left and a factory Toyota unit on the right. If you’re not familiar with how a starter works, notice the small gear visible at the top of each unit. When you turn the ignition key to start the engine, that gear slides forward and engages the flywheel behind the engine, and spins it rapidly to enable the ignition to catch and start the engine. Once it starts and you release the key, the gear slides back out of engagement.
It should be obvious that that gear is subjected to a great deal of stress—which is why the factory starter has a nose cone that supports the end of the shaft on which the gear slides, hugely increasing its stiffness (and also possibly helping keep random dirt and debris away from the shaft and gear).
Now look at the aftermarket starter. No nose cone, no support for the gear. Cheaper to make, for sure.
Which would you expect to last longer?