It happens because of cheap parts and/or normal variances.
For example, Sony is well known for using the WORST LASERS IN THE BUSINESS in the Playstations. Sony knows full well that under heavy use the average PSX or PS2 ... WILL FAIL ... within a couple years. Every single one. There are some that survive, but not many.
Go look on eBay. It's FULL of PS2's. And MOST of them have bad lasers, although the ad often won't tell you that.
It has to do with cost-cutting, usually. Once in a while it's because you just get a bum laser. But most of the time it's cost-cutting. Lite-on is a budget brand. For a short time they were also the best rippers available, and the best burners available. This was right before DVD burning became all the rage. The last-gen of CD burners... Liteon RULED. That pretty much cemented their place in the lineup. But they've NEVER been "well built" drives. They've always been the cheap alternative. Back then we used to tell people that for the price of a single Plextor burner they could get two (sometimes three!) Liteon burners... and keep the extra one as a spare in case the first one burnt out! LOL!
Essentially some bean-counter somewhere did some analysis and determined the minimum part specification necessary to ON AVERAGE last at least a year (the usual RMA/Warranty period the OEM's provide). Some manufacturers use a nicer part (Plextor in the past, Benq now, Pioneer). Some use THAT PART AND NOTHING NICER EVER (Liteon, Sony).