Traccar supports many protocols so you are usually able to buy a random tracker and reconfigure it to work with your traccar instance.
When I think of this space I have have four thoughts.
1. Ordinary users don't want their location data used by third parties. Which eliminates all of the commercial offerings.
2. They also don't want to pay $50/mo for someone to track them and then turn around and sell their data to everyone.
3. The three functions people want are a) Someone just drove off with my car detection. b) Where did the thieves leave my car. c) Is my car going to get a parking ticket at 6am tomorrow.
4. And what stops me. Spousal abusers and stalkers misusing the technology. You can solve 1, 2, 3 by a device that doesn't report it's location unless pinged remotely, it leaves a 'location box' like your driveway, or if someone tampers with it. I can't see how to solve #4. Maybe it doesn't matter because stalkers probably want a tracking device not a locator device.
Traccar was awesome for this.
It also might have been more expensive to design and produce their own PCB with all these features, so if the Pi is cheaper, why not use it?
Also, there are a ton of information, tutorials and libraries available made for the Raspberry Pi. Now their customers can take advantage of all that too.