Because there is no newly invented architecture called "modular monolith" - monolith was always supposed to be MODULAR from the start.
Micro services were not an answer to monolith being bad. Something somewhere went really wrong with people's understanding and there is bunch of totally wrong ideas.
That is also maybe because a lot of people did not knew they were supposed to make modules in their code and loads of monoliths ended up being spaghetti code just like now lots of micro service architectures end up with everything depending on everything.
Micro services were not an answer to monolith being bad. Something somewhere went really wrong with people's understanding and there is bunch of totally wrong ideas.
That is also maybe because a lot of people did not knew they were supposed to make modules in their code and loads of monoliths ended up being spaghetti code just like now lots of micro service architectures end up with everything depending on everything.