Why is there a commandment against merely coveting¹ -- something that happens exclusively inside the mind and can, in fact, serve as inspiration to achieve? I'd appreciate your input, Steve

Answer:

The mind is not only where all our actions begin, but it is a valued realm in its own right. The mind is much more sophisticated and significant than the hand for example. So if we have commandments that regulate the actions of our hands, it would only make sense to have commandments that regulate the mind. 

Not all coveting is bad. The commandment specifies what type of coveting: physical possessions and desires. It is OK to covet someone's positive achievements or spiritual accomplishments, since it can inspire you to achieve your own. Additionally, emulation and inspiration is not the same as coveting. One can emulate someone else and be inspired to accomplish something without actually coveting what belongs to the other person.