That's because the batteries have become the printer ink of the tool world. They're f'n expensive.
If you buy into a product system it makes no sense to have different batteries that don't fit all the tools. If you keep the batteries all the same then you can be charging one or two sets vs having to buy extra sets and charger multiplied by the different tool makers.
I have one of the manufacturers shown in the image, and after I got a kit that had a charger, tool, and extra batteries included I got hooked in because they sell tools without batteries, but I have extra! So I bought same maker. The tools are all pretty good, so not much difference between makers, but that's one way they hook you.
The batteries are actually really simple. The best thing you can do to increase efficiency and save money is learn to replace the cells. You can get higher quality lithium cells than the batteries come with if they start to fail for way cheaper than you'd expect. The boards have like 12 circuits, and if you can't fix them you can buy them for a few bucks.
I’ve done that on older battery packs. It’s a PITA because many packs are sealed, not screwed together, and you have to physically cut them apart to get to the batteries within for replacement. And assemble the batteries, which are often soldered in order. Then you gotta figure out how to reassemble the whole thing in a way that it won’t crack apart again. Doable, but definitely not for everyone.
This is the way and how they get everyone hooked. My husband was -this close- to buying a badly reviewed lawn mower all because he had 3 batteries from the same brand. Two hardware store employees talked him out of it even though it meant my husband walked away with a cheaper brand, bless them.
This is the entire motivation I had to buy what I did when I became a homeowner. I don't want a random assortment of batteries and chargers strewn around the place, especially when a replacement battery pack can run upwards of $300-500 depending on its capacity.
I chose Dewalt, but mainly because I had a pretty capable little DeWalt 12v cordless drill for a long time and I was very happy with it and the durability it had... I purchased a pack of tools, and switched to the 20v "Max" (or whatever variant is local), which is also compatible with the flexvolt, which is good because we added a few garden tools (string trimmer, hedge trimmer, and later a lawnmower). All of the tools we have use the 20v batteries. As luck would have it, DeWalt also released a snow blower in the last few months, which we promptly purchased, since where we live we get quite a bit of snow, and nobody wants to shovel. The only catch is that it uses the 60v flexvolt batteries; the flexvolt batteries can work at 20v, so those batteries can be used on any other tool, however, since the blower is 60v, only those flexvolt batteries can be used with it. It's the only tool that requires the higher voltage.
There's a lot of yellow tools.
I don't fault anyone for using a different brand; this is just what I am using.