There are lots of reasons - you should read this.
1) Power requirements
While you will see internal batteries for some single axis rigs, or lighter multi-axis rigs, internal batteries don't hold up for the payloads, speeds and performance of the spectrum ST4.
The spectrum ST4 is very power efficient, and faster, quieter, and supportes heavier payloads than anything with internal packs.
If you are only looking for timelapse solutions, there are a few rigs out there that might do what you want. You will find they have fewer axis of control (driving one motor is easier than driving 4), are noisier (driving motors quietly and powerfully takes energy), and slower (more speed more amps) than the spectrum ST4.
As we fit more and more into professional shooters toolkits, we have adopted integration to both VMount and Gold Mount batteries - industry standards.
A 95 WH pack will reliably drive your spectrum for 6 hours with all axis moving continuously with heavy payloads.
TSA and air travel
If you have internal batteries, you must carry the equipment on board. You cannot check or your equipment can be confiscated. For professionals, this is not an option.
Batteries are getting better, but they wear out. Having internal batteries tied to a multithousand dollar piece of equipment is a sure fire way to expire that equipment after a period of time unless you pay the manufacture to replace the pack.
eMotimo spectrum ST4's are built to last, not expire. What camera manufacture these days has an internal battery that is not user replaceable?
Hoverboard, iPhone, Samsung Note. Dell. Fires, injuries, big issues with TSA.
Batteries and battery design is tricky. Even with great design teams from massive companies, manufactures can have defects that result in fires / explosions. If you are buying a lithium ion pack that has a few cells and decent capacity, there is a risk of fire with drops / damage, or through issues with suppliers.
No motion control company that we are aware of has paid UL (underwriters's laboratory) to evaluate and approve their designs. We much prefer to encourage users to use UL approved packs that have been fully tested. Buy name brand packs that have been fully tested.