If North American stock markets are semi-strong efficient, why are indices like S&P500 expected to grow?
The composition of a stock market index is not fixed but changes over time. Moreover, it usually changes so as to increase/decrease the weight of good/poor performers.
For example, the S&amp;P500 assigns weights to 500 large companies that meet various criteria, including at least a $8.2B cap.* These weights change over time. In fact, companies can even be dropped (e.g. recently Macy's) or added (e.g. Netflix, Google, and Facebook were not on the index in 2000 but are today).
So, suppose we're transported back 20 years to 2000-11-06. Consider the 500 stock prices that were included in the S&amp;P500 on that day. It is true that the EMH would have predicted that those 500 stock prices would, on average, not have grown in the following 20 years (ignoring inflation). However, the EMH would not have said anything about the S&amp;P500 Index, whose composition may and indeed did change.
(We can repeat this same thought experiment for the 500 stock prices included in the S&amp;P500 today: The EMH predicts that, ignoring inflation, these 500 stock prices will not change over the next 20 years. However, the EMH says nothing about what the S&amp;P500 will be in 2040.)
So, if we expect economic growth and creative destruction, then we also expect the S&amp;P500 (and any other index whose composition is allowed to change) to keep growing over time, even if we do not expect any individual stock price to grow.

*See e.g. &quot;S&amp;P U.S. Indices Methodology, Aug 2020&quot; PDF.