@Reasons for not learning a language:

The more languages you learn, the more access you have to humanity's written riches. That includes newspapers, research papers, literature, scientific and otherwise. Relying on translators to bring that material to your language is a dicey proposal, especially if you're looking for something very specialized. It's also a lot more comfortable to read stuff in good Native Language than in horribly butchered English, which is all too often the case with science papers (not that scientists as a whole tend to show much writing skill in the first place, it's one of those "unnecessary" social skills that are "beneath them"). Not having to translate contracts and other legal documents, and having direct access to all documentation, can give you a significant edge both in time and in administrative fuss.

You also have access to an increased wealth of human contacts, and you will endear yourself to them considerably by showing that you learnt their language.

Now sure, you can get by with English almost everywhere... You can also live on instant noodles, technically. It's still not a good idea to try it if you want to have fully efficient access to the information, tools, and people, that lie in the most scientifically brilliant and economically powerful countries besides the USA and the UK.