This isn't a legal definition, it's the dictionary term
>(the act of) dishonestly taking something that belongs to someone else and keeping it.
How is this action of taking someone else's idea and keeping the fruits of their mental labor not thievery?
Again, not legally arguing. The point here was to exercise the psychological reason that having your ideas taken without permission nor even acknowledgemrnt feels bad. Theft is bad and feels bad. People want to avoid that where possible.
That's not remotely true even with today's absurd laws.