batteries have much higher profit margin...

computer manufacturers want them to last acceptably long but not too long :)

quality being equal, practical battery life is largely affected by cell compatibility (discharge/charge characteristics) and charging process/algorithm.

i don't think that memory effect exists any more. it's just normal or accelerated battery deterioration.

heat kills battery fast that's why laptop batteries don't last long.

deep-discharge can permanently kill Lithium-Ion batteries that's why protective IC is available in all Li-Ion battery packs with more than one cell. It's not possible to deep-discharge them because protective IC cuts off when voltage drops to certain level.