Because you think matter and antimatter annihilation will result in nothing.
It doesn’t. I think you are misunderstanding the annihilation bit.
Annihilation doesn’t cause nothing: it will always cause something: eg radiation, heat, energy.
There are still the matter of conservation, where mass or energy are neither can be created, nor destroyed, BUT transformation can occur with one or the other...meaning mass can convert to energy, and vice versa, energy can convert into mass.
And this would still apply to annihilation. So, something will turn into something else, or something don’t completely cease to exist, but changes will occur.
For instance, the electron-positron annihilation will either produce and release as...
(A) W bosons or Z bosons
or (B) as gamma ray photons.
The electron and positron may not exist as the same particles before the annihilation, they will turn into one of the above particles (eg A or B)
And it is the same with every other particle-antiparticle annihilations, they will all produce different particle and energy.
The release of these particles are not “nothing”, so your claim only demonstrated that your understanding of the particle physics is seriously lacking.
What I don’t understand is why you would make claim that are not true when you don’t actually know what you are talking.