Why don't nimh's need balancing?

Back in the mid 80's when we raced with the "hot" 1500 and 1600 SCE, SCR NiCds we used to balance them. The hot ticket was the "Cell Mate" charger. Every month/end of season we'd dissassemble our packs. It would charge the cell and give you all the readings. Hi/lo/avg/disc rate, etc. Keep notes and build your packs according. The "notes" were like the stickers you see on the NiCd cells these day. I still have a couple of theses packs and surprisingly one day I charged/cycled a couple and they did pretty good in my 'Pede (when it was brushed w/a Reedy) We used to time/gear for 6 minute races then too IIRC. Hell, when we raced "stock" class they stilll had fixed/crimped endbells. The only mods you could do was play with the brushes and springs.