Exactly, and it is a shame. Stability and building on the shoulders of giants is how progress is made. Not by rewriting everything all the time.
Luckily I don't work on frontend UI stuff. I learned UNIX, kernels, cryptography, TCP/IP, SQL and similar technologies in the late 80s to early 90s. While all of these areas have evolved and progressed, it's been a gradual incremental change year to year. A textbook on any of these topics from 1990 is still recognizably relevant, even where details have changed over the decades.
I've observed similar technology shifts with backend code (where MySQL was hot in 2003, PostGres in 2006, MongoDB in 2009, Cassandra in 2011, PostGres again in 2015, and now there's this huge explosion of storage solutions) and in platforms (where we were all about the web in 2007, all about mobile in 2010, all about blockchain in 2013, all about smartwatches & VR in 2015, and all about Ethereum & smart contracts in 2018).
That's exactly the same web-dev mindset, only applied to backend code. I find it remarkable that you take a half-life of 12 months to be a given, rather than a screaming red flag.
Yes, the world changes, and code needs to change along with it, but it doesn't change that much. Replacing 50% of your codebase every 12 months, year after year, indicates to me that the organization is just replacing poorly architected code with different poorly architected code, not better code. The codebase is on a random walk, and any forward progress it makes is due to chance and evolutionary pressure, rather than reason and design.
Turnover is definitely not uniform and a 12 month architecture lifespan is not, in my experience, either normal or healthy.
My experience has been that paradigms and tools can move fast and are relatively straightforward to pick up. Deep understanding of the underlying problem domain and the systems underneath the abstractions is harder to pick up, but the concepts move more slowly.