Why can you not plant your own roundup ready seed that you grew?

Assuming you received the original seed legally, you signed a legally binding agreement with the supplier (and, by affiliation, with Monsanto Co.) that you would not do so. Monsanto wants to protect their research investment and so have come up with this method to try to prevent anyone stealing their technology.

Other than the legal agreement, there is no botanical reason why you couldn't plant your non-hybrid harvested seeds such as soybeans. Harvested hybrid corn, though, will not produce the same plant as the original F1 hybrid. You would not be very pleased with its field performance, though it would still be glyphosate-resistant.