Why don't battery manufacturers make 5 V batteries?
They don't have a choice

The voltage of a battery is decided by the reactants in the battery.

There are only so many viable battery chemistries out there.   You can't just pick two random chemicals off this table; it also has to be possible/practical -- 


To actually build it. 
At a competitive price-point.
Out of readily available materials.
Which are relatively non-toxic.
And don't weigh too much. 
Endure many recharge cycles (if it's secondary).
Never explode.
Have enough storage capacity to bother.
and not hopelessly lag behind other successful batteries in any category. 


These restrictions soon winnow the thousands of combinations down to just a few, and none of them have a 5V or 2.5V output.