[JBorrow]

There are other things that must be conserved - not just energy - in the current standard model of particle physics. For example, you have to conserve charge - and matter and antimatter have opposite charges.

In that case, say I have 100 electrons that I want to turn into positrons. If I was able to turn these into pure "energy", first I'd have to figure out how to give that "energy" charge (so it could conserve charge in that step), and then I would have to create some other particles to balance out the 100 positive charges that the positrons would give me, ending up with 300 particles.

The reason that we think that there's more matter than antimatter in the real universe is because of a thing called "Charge-Parity Violation" [1] that is the focus of a lot of current research.

[1] https://www.symmetrymagazine.org/article/charge-parity-viola...

[paulddraper]

Momentum is an example of another conserved quantity.

[MrEldritch]

Incidentally, fun fact - you already know that X, Y, and Z linear momentum are all conserved separately. In special relativity, mass-energy conservation gets folded into this as well - an object's mass-energy is just the component of linear momentum along the time axis! (and "rest mass" is its value in the reference frame where the object is otherwise stationary and moving only through time.)

[antidesitter]

This is conservation of 4-momentum, whose spatial components are ordinary momentum and whose temporal component is energy. By Noether’s theorem, conservation of 4-momentum is due to the invariance of physical laws under 4-translations (spatial and temporal).