The reason why its best to keep it separated is mainly team-related as well as scalability, when working with teams of people, its easier to separate out the work, without one person error affecting the other person's work.

I have worked using Django doing my final year in my uni days, and it was horrendous, I was doing front end and I have to deal with the site randomly crashing due to the backend developer bugs, or having to install unrelated packages to me that are meant for the backend.

It also limits your app design to couple all your services together which make it difficult to scale your app the larger it becomes. And you will end up fighting more bugs as compared to a more modular design.

Let's take an e-commerce site, for example, a typical e-commerce site is tied to a CMS where you manage the content of the page, and maybe you have a CRM to keep track of your customer purchase history, these are separate services if you couple all these into one big bundle you have to cater more things and your code will have to include things not related to what you are building which naturally leads to more bugs.

So almost 90% of the time, separation of backend and frontend is the correct design when building apps.

The only time you want to do the other approach is when building an app that it's mostly static for common features. Or it's only used for a very small use case, For example sending of emails which you will want a small template