Talking about both photons and electrons. Uncertainty principle is about position and momentum, but electron is considered wave?
We have mathematical model of light being EM Waves, what does it being particle even mean?
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Everything we call a particle exhibits wave-particle duality at some scale. This is at the heart of quantum mechanics. For something like photons, the wave-particle duality is easily seen through the double slit experiment. If we let a few photons through at a time, we will see where those individual photons strike the screen in the end. If we let a lot of them through, we will see a wave-like interference pattern emerge. So it’s not that the electron “is considered a wave.” The electron is a particle, it’s just one that often prefers to behave like a wave, which is by no stretch unusual.
The degree to which the wave or particle nature is more/less noticeable depends on a lot of factors, such as the types of particles you’re working with (and their properties), and whether/how they’re entangled with other particles or the environment. In lighter particles–like massless photons or very light electrons–the wave-like properties tend to dominate at the relevant scales, and we usually only see particle-like behavior when we cause some kind of interaction/measurement that forces the particle to take a position. In the case of the double-slit experiment, the photon would continue to evolve probabilistically if we didn’t force it into a position by interposing the screen.
We have found through a lot of experimentation (old and new) that heavier particles also exhibit wave/particle duality, but there’s a limit past which the wavelike properties aren’t really detectable or relevant. A particle’s “de Broglie” wavelength is the wavelength scale at which a particle’s wavelike properties become important. But when you start to bind simple particles together into larger particles/systems/compounds, you very quickly reach a point where the size of your particle/system/compound is *larger* than the de Broglie wavelength, at which point the wavelike properties of the particles are no longer noticeable or particularly relevant.
If you want to ask “why” questions, you’re getting into foundations of QM which is probably outside the scope of ELI5.
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