They might & I can definitely see why this would be helpful 
But thereās a few reasons why Monzo might want to keep chat as app only..
Firstly, when you connect to the chat, youāre connecting from a ātrusted deviceā so Monzo donāt need you to answer all of the annoying security questions that weāre used to the legacy bankās asking.
Secondly, Monzo are able to use machine learning to predict what your query will be, based on the last couple of hundred actions you took in the app. That then feeds through to the help topics that you see in the Help tab & to the COpās suggested solutions which should help them answer your question faster. Maybe that data could feed through to the website but then youād need to login in order for it work apply to the help page. Which looks a bit like this -
Slide from this talk.
Lastly, Monzo are planning to add actions to help -
so youāll soon be able to solve your issue directly, if youāre in the app 
Even if Iām eventually wrong about this, hopefully this post will highlight some of the cool thing that Monzoās doing in this area for users who havenāt heard about them before 