I work in what used to be Exchange. (my opinions are my own).

There is no one reason for the quality issues. It's a thousand small decisions and problems that have compound against each other over decades, coupled with the sheer feature complexity+scope+impact and multiplied by the titanic scale and volume the platform handles.

Additionally, the engineering culture really prioritizes backwards compatibility for customers (for good reasons) which bleeds into all aspects of the platform/decisions in both good and bad ways - and means that the big and obvious step-change platform improvements that could be made internally to make things better are not really invested in, or are deemed to expensive.

It's still a great place to work, and I'm proud that my work is in some small way directly contributing to and helping billions of people's work lives but there's still a long road ahead to improving the customer experience of using the platform for both internal and external customers.