Moreover, Meccano taught important lessons that Lego could not: understanding engineering tolerances. Lego bricks just snap together, and unless you are building something pretty infeasible that's generally the end of it. Meccano was all about lining up plates and brackets by eye (the holes were bigger than the screws), making sure things weren't too loose or too tight, ensuring that load-bearing parts were properly cross-braced, and so on.
On the other hand, Meccano was pretty perishable. It didn't take long to scrape the paint off the parts and permanently mangle the so-called flexible plates.