Meccano is 120+ years old, so no patents to worry about there.

Lego's robotics stuff might be patented but their core brick isn't anymore. However the generic replacements like Mega Bloks just don't snap together nearly as well as LEGO. For a toddler making a little garden out of a few blocks it's fine, but you can easily make a vertical Duplo stack that's 3 times taller than the maximum stack you can make with Mega Bloks, and that kind of structural integrity becomes very important in robotics.

This was the thing that struck me when I ran a Mindstorms summer camp a bunch of years ago: The kids were learning to program the brick with a programming language, but the physical design of the robots they made was, itself, a kind of 'physical programming.'

And legos, more than most things, are thus programmable structures. Recall that lots of early computers (for example, the one on Apollo missions) were NOT re-programable. The code was encoded in the physical form, and couldn't be moved around without essentially re-manufacturing the device. The step to programable computers was a major milestone.

Legos make a similar leap in an engineering space instead of a computational space; it's super cool.