there are many good answers in this thread ,but I haven't seen this point:

More batteries = more connections

My first rule of troubleshooting is "90% of the problems are in the connections". You'd be surprised how many 'dead' TVs and PCs come back to life when the power plug is taken out and re-inserted. Each connection is another point of failure in the system. If you had ten batteries, you'd have ten times the chance of a failure.

And presumably, you'd have the consumer opening the hatch and changing the battery. I can tell you, the engineers in most tech companies would hear "consumer changing the parts" and have a heart attack. It's a recipe for a host of problems.