Citing "The overall mission" from #26:
This language contains the bare minimum of constructs to allow for expression of rendered HTML code [...] and only the most simple calculations [...] to avoid the creation of components that attract logic
The existence of arithmetic operations will almost definitely lead to the attraction of logic in components. It is therefore better to remove them from the language core.
If legitimate use cases for arithmetic operations should arise in the future, it will still be possible to add them back. For now it'll be easier to develop the language without them.