What BIGRED said -- though having bought multiple tools that way (as tools only) I can appreciate their selling them without the battery as the lithium batteries for each tool brand tend to be ridiculously expensive (e.g. the highest capacity batteries for each brand tend to cost more than the cheapest tools that use them ...some times 2x to 3x the price of the absolute cheapest tool in the line).
It's enough that most people will stick to one or (maybe) two brands of cordless power tools since the batteries aren't only unique to the brand, but the power level (e.g. one brands 18V batteries only work with that brands 18V tools) so even in a single brand there is usually a few different battery lines. Usually one for 18V/20V tools, and one for 12V tools - though some brands will also add in other battery voltages as well (e.g. Milwaukee) - or have some batteries that work with multiple voltages (e.g. Dewalt's flexvolt 20V/60V
While it's possible to find adapters to cross brands I'm not sure how much I'd trust any of the adapters. It's usually easy enough to watch for sales & specials (especially after signing up for the respective companies email advertising) -- I have gotten a fair number of spare Makita batteries through "buy a combo kit and get an extra two batteries free" promotions.
Thankfully though the batteries on all brands tend to last long enough and charge quick enough that most people won't need more than two or three batteries (perhaps a few more if they have tools that need multiple batteries at a single time) plus a charger. Swapping a couple batteries back and forth between a tool and the charger can keep a person working for a very long time and indefinitely if the charge time is less than the time to run the battery down.
