Can be*, that's not necessarily always true. I've quit jobs plenty of times without having any plan for the future or particular drama-reason for leaving, just "It's not as fun here anymore, despite this being a great place to work", I'm sure I'm not the only one who does so.
What I've never done though, is leaving a place without being 100% honest exactly why I'm leaving. I won't say "I was just ready for change" if that wasn't the reason, I have no reason not to be honest about why I'm leaving.
I've generally had 10+ year tenures other than a return to school that was basically always in my plan and dot-bomb (leaving a company I wasn't really a fit with anyway). But, yeah, I've always been ready to move on at about that ten year point which is actually fairly long by a lot of people's standards in the tech industry.
I do disagree though that, unless there's some actionable change that would specifically benefit you like more money, my answer outside of private conversations with people I know well is going to be some variant of "time for a change." Anything else just invites arguments and conversations I don't want to have.
Can be*, that's not necessarily always true. I've quit jobs plenty of times without having any plan for the future or particular drama-reason for leaving, just "It's not as fun here anymore, despite this being a great place to work", I'm sure I'm not the only one who does so.
What I've never done though, is leaving a place without being 100% honest exactly why I'm leaving. I won't say "I was just ready for change" if that wasn't the reason, I have no reason not to be honest about why I'm leaving.