Are there theories that explain wave-particle duality?

Yes, several actually.

As anna v explained in her reply, at the most fundamental level the only answer physics can give to the question Why? is Because!.

It's impossible to give a more meaningful definite answer until we've taken another step down the rabbit hole (assuming we've not yet arrived at the bottom, of course).

However, you're not the only one uncomfortable with the situation, and that's where interpretations of our physical models come in, and there's a whole bunch of them for quantum mechanics.

As far as wave-particle duality goes, here are the explanations given by various interpretations:

• according to the statistical interpretation, quantum mechanis only describes ensembles and the question cannot be answered without an underlying new theory
• according to the de Broglie-Bohm interpretation, there are both particles and waves, the former being guided by the latter
• according to the transactional interpretation, we have waves going forwards and backwards in time, resulting in particle-like interactions by interference
• according to the many worlds interpretation, we have particles going all possible ways in different worlds
• according to the consistent histories interpretation (on which I should probably read up some more, so take this with a grain of salt), the particle or wave-like characteristics are just artifacts of a particular choice of history, which gain reality be decoherence

The Copenhagen interpretation is missing from this list by intention ;)

Are there theories that explain wave-particle duality?

Yes, several actually, but they are interpretations rather than new theories.

As anna v explained in her reply, at its most fundamental level the only answer a physical theory can give to the question Why? is Because!.

It's impossible to give a more meaningful definite answer until we've taken another step down the rabbit hole (assuming we've not yet arrived at the bottom, of course).

However, you're not the only one uncomfortable with the situation, and that's where interpretations of our physical models come in, and there's a whole bunch of them for quantum mechanics.

As far as wave-particle duality goes, here are the explanations given by various interpretations:

• according to the statistical interpretation, quantum mechanis only describes ensembles and the question cannot be answered without an underlying new theory
• according to the de Broglie-Bohm interpretation, there are both particles and waves, the former being guided by the latter
• according to the transactional interpretation, we have waves going forwards and backwards in time, resulting in particle-like interactions by interference
• according to the many worlds interpretation, we have particles going all possible ways in different worlds
• according to the consistent histories interpretation (on which I should probably read up some more, so take this with a grain of salt), the particle or wave-like characteristics are just artifacts of a particular choice of history, which gain reality be decoherence

The Copenhagen interpretation is missing from this list by intention ;)