Starting a habit from a blank slate means relying on willpower every single day to remember and act - that's a harder, more fragile system than one built on an existing routine.
When you link a new action to an anchor habit, the anchor does the remembering for you.
Compare the two: a cold-start habit needs you to notice the time, decide to act, and follow through - three separate moments where you could drop it.
A linked habit collapses that into one moment, because the anchor habit has already got you moving - you're just adding a second action onto momentum you already have.
That's not a guarantee of success, but it does remove one of the biggest points of failure: forgetting to start at all.