Why does a US/Canadian STEM PhD 'almost' always comes with monetary assistance while a British PhD doesn't? A US/Canadian STEM PhD 'almost' always comes with monetary assistance from the supervisor. In some universities/programs, supervisors are able to individually recruit students along with monthly allowances. In most cases, students don't even need to submit a separate application for money.

Even PhDs in Hong Kong and Singapore come with full funding. Students don't even need to submit a separate application for money.

Why doesn't that happen in  UK?
It does, supervisors in the UK can support PhD candidates with funding - based on the research projects they are recruited to...

Some PhD's have funding through external companies especially those in some engineering disciplines - usually found in universities that have strong links with industry such as research involving stress &amp; structures (I remember some funded by Westland...)