People who are interested in this topic - what REST is and is not, what it's meant to be, and so on would do well to read Fielding's thesis in full [1]. There is quite a lot of misunderstanding of the topic.
REST as a concept was created as a way of characterizing a successful architectural style seen in web applications, a common set of constraints and advantages realized from those constraints. REST is attempting to characterize the architecture of the web, so I think it would be accurate to say "the web itself is RESTful":
> The first three chapters of this dissertation define a framework for understanding software architecture via architectural styles, revealing how styles can be used to guide the architectural design of network-based application software. Common architectural styles are surveyed and classified according to the architectural properties they induce when applied to an architecture for network-based hypermedia. This classification is used to identify a set of architectural constraints that could be used to improve the architecture of the early World Wide Web.
> Architecting the Web requires an understanding of its requirements, as we shall discuss in Chapter 4. The Web is intended to be an Internet-scale distributed hypermedia system, which means considerably more than just geographical dispersion. The Internet is about interconnecting information networks across organizational boundaries. Suppliers of information services must be able to cope with the demands of anarchic scalability and the independent deployment of software components. Distributed hypermedia provides a uniform means of accessing services through the embedding of action controls within the presentation of information retrieved from remote sites. An architecture for the Web must therefore be designed with the context of communicating large-grain data objects across high-latency networks and multiple trust boundaries.
> Chapter 5 introduces and elaborates the Representational State Transfer (REST) architectural style for distributed hypermedia systems. REST provides a set of architectural constraints that, when applied as a whole, emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. [...]
> Over the past six years, the REST architectural style has been used to guide the design and development of the architecture for the modern Web, as presented in Chapter 6. This work was done in conjunction with my authoring of the Internet standards for the Hypertext Transfer Protocol (HTTP) and Uniform Resource Identifiers (URI), the two specifications that define the generic interface used by all component interactions on the Web.
(To be clear, I'm not intending to comment on whether REST is good or bad, nor its suitability to a purpose like API design. I just mean to comment on what it factually is and is intended to be, since from my perspective it frequently seems to be misunderstood.)
[1] https://www.ics.uci.edu/~fielding/pubs/dissertation/introduc...