We all designed and architected our monoliths with the best of intentions. We used all the best frameworks, patterns, tools, techniques, and practices. All these may have kept up with the times, more or less, but they are all pretty much rooted in the context of more than a decade ago. Back then infrastructure took months to provision, releases were measured in quarters at best, and deployments were performed manually. Everything about that context incentivized the monolith.
The frameworks we used, such as dependency injection and object relational mapping, were designed to solve problems in the context of the monolith. For without these levels of abstraction it is very difficult to evolve a monolithic architecture. When everything runs together then everything must evolve together. Certainly we could branch-by-abstraction to evolve the functionality...