Roads and public transport are two primary responsibilities of government. Everybody uses roads. A minority use public transport. I see this a a metaphor of sorts for shared infrastructure versus shared services.
The roads provide the infrastructure that everyone can use. They aren't necessarily complex although advanced functions can be more so. They aren't all that hard to join together although if different standards are used, that connectivity can be challenging but it can be overcome. It can be easy to start a simple road, only one lane, not sealed, not even necessarily level. Similarly, a single network can be easy to establish. But, if everyone is building them, efficiency is quickly lost. These common tasks are better done centrally to generate economies of scale and to increase standardisation and reduce costs.
To continue the metaphor, public transport is an application that uses the road infrastructure. There are many other such applications, large and small. Some do the same things in different ways, some do different things in the same way. Some people can afford specialised vehicles or choose to do so. Others look for the cheapest or most efficient alternative and are prepared to travel together.
The use of public transport requires some compromises. One travels according the schedule, sits in an assigned seat, pays for one's ticket, etc. Some advanced vehicles (applications) like aeroplanes (think ERPs) require such specialised handling and maintenance that almost everyone uses them.
However, even at that level, people can go it alone. Sometimes, it takes impositions from above to make people use public transport - think buses to special events. When the purpose is common, the choices are restricted, and/or the price is right, people will use these options.
Thus it is with shared services - sometimes more factors need to align to make them work than is the case for shared infrastructure.