They don't. Most NiCads don't either, except sintered plate NiCads do (if anyone still uses them) but you have to: 1. discharge them to the exact same point each time for it to happen (very hard to do) and: 2. not overcharge them at the end of a charge cycle, but most chargers do this so they know when to shut off (they dectect the slight voltage drop when a cell is overcharged).

I think most people are confusing 'voltage depression' with 'memory effect'. Voltage depression happens with NiCD and NiMH batteries with age. Each cell may drop fairly quickly from 1.2v to 1v, for example, but then it levels off and the charge still lasts it's normal time. This can be cured by completely discharging each cell individually and then charging it again. But, if you do this to the whole cell pack (which most people do), some cells will discharge before others, and the discharged ones will then be partially recharged backwards by the stronger ones in the pack. This is not good and can kill the pack.
The older a battery gets or the more times it's charged, the less it's likely to hold it's charge. In other words, batteries do have a life span. For Lithium batteries, estimate that after 50 charges they will likely have 50% to 80% of their original performance (depending on how they've been treated/abused). Some new LiPo batteries are claiming to be good for 200 charge cycles. You'll know which ones they are because they make a big deal about it in their advertising.

Is this enough information?