There's a great deal of overhead when communicating between services that isn't typically present in a more tightly-integrated monolith. Networks are very, very slow relative to RAM, setting aside the other costs involved in serializing/deserializing every communication.
And "properly designed" is very hard to achieve.
It's often much more productive to start with a monolith and experience the pain of extracting a piece that needs horizontal scaling, than to start by horizontally scaling everything.
Microservices can be a good architecture, but it's far from a no-brainer, even for high scalability. Servers are incredibly fast these days and can typically scale very well vertically.