Why do some geographers consider Europe and Asia to be one continent called Eurasia?

'Europe' and 'Asia' are socio-political continents, they're 'different' because people, for different reasons throughout history, have decided so. It was the ancient Greeks who first decided that 'Europe' and 'Asia' were two separate 'lands', though the exact border between the two was still unclear.

When talking about physical geography though, it is all one land mass, it is all one physiogeographical continent, regardless of the people who live on it and the divisions that have made.