Why was it that past mathematicians researched in multiple fields?

Question

Looking at biographies of past mathematicians, I notice that these biographies state that these mathematicians had contributions to things like mathematics (obviously), physics, chemistry, biology, medicine, etc.

did mathematicians today stop doing multiple fields? when did this stop? or is this still commonly practiced today?

Answer 1

H. Poincaré is often cited as the last one

Henri Poincaré was a mathematician, theoretical physicist and a philosopher of science famous for discoveries in several fields and referred to as the last polymath, one who could make significant contributions in multiple areas of mathematics and the physical sciences.

See https://plato.stanford.edu/entries/poincare/

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update:

I do agree with comments concerning von Neumann, Kolmogorov... Just wanted to be clear, I am not personnally saying that Poincaré was the last one, but that he was often cited as the last one. Hence I am not the one you must convince if you do not agree :)

H. Poincaré was also often cited missing rigor: La rigueur mathématique chez Henri Poincaré (in French sorry)