Other architectural characteristics dominate
Evolvability is only one of many characteristics architects must weigh when choosing a particular architecture style. No architecture can fully support conflicting core goals. For example, building high performance and high scale into the same architecture is difficult. In some cases, other factors may outweigh evolutionary change.
Most of the time, architects choose an architecture for a broad set of requirements. For example, perhaps an architecture needs to support high availability, security, and scale. This leads towards general architecture patterns, such as monolith, microservices, or event-driven. However, a family of architectures known as domain-specific architectures that attempt to maximize a single characteristic.
An excellent example of a domain-specific architecture is LMAX, a custom trading solution. Their primary goal was fast transaction throughput, and they experimented with a variety of techniques with no success. Ultimately, by analyzing at the lowest level, they discovered the key to scalability was making their logic small enough to fit in the CPU’s cache, and preallocating all memory to prevent garbage collection. Their architecture achieved a stunning 6 million transactions per second on a single Java thread!
Having built their architecture for such a specific purpose, evolving it to accomodate other concerns would present difficulties (unless developers are extraordinarily lucky and architectural concerns overlap). Thus, most domain-specific architectures aren’t concerned with evolution because their specific purpose overrides other concerns.