In the case where you have loose coupling but are representing multiple entities that scale in different ways, microservices allow you to separate concerns and separately scale those concerns relative to their requirements in terms of memory/CPU/disk/network/etc. The best factored code running in a single horizontally-scaled layer will be inefficient if 90% of requests are manipulating entity A, and entities B, C, and D have a lot of intricate business logic but are rarely touched (they are better off if separated and scaled individually)
The overhead you allude to is definitely something to take into account. If you're a 5-20 person startup without a serious need to scale up or lacking people who have built the tools that make microservices easy, you should avoid the issue for now. But ultimately, decoupling services so they can horizontally scale independently is a huge win.