It's more likely ignorance of your specific market price than malice.
First, the employer isn't even going to know your outside market price if you don't get an offer in the first place. For you to prove that you're worth the 10% salary increase, you would have to be vetted by a prospective employer as having met their requirements to be hired at some price.
Secondly, your current employer needs to agree that you're worth that 10% salary increase. Their perception of your value may simply not match your own perception of your value.
In these situations often the employee is getting raises that do not even cover inflation. So the employee now sees their market value and would like to continue working for the company, but they have realized they could take on risk for a 20-50% raise instead.
I mean the risk equation is simple, with such a raise you can save enough cash to justify the risk even if it doesn't work out.
I always ended up taking the offer from the more generous company, and they tended to increase my pay to offset more than inflation and then a bonus to justify staying.