Perhaps for the same reason Jesus is saying the same thing in Luke?
But FK will tell you these never really believed. To which I say, but they claimed they "knew" they were saved until they were tempted.
Sure, it's easy to be "certain" when things go well. True Christians showed their colors when things got rough.
Isn't that what the story of Peter betraying Christ is all about: those who thump their chest and claim how they "know" they believe and are saved, and would never leave God, are the first to do just that?
34Jesus said to him, "Truly I say to you that this very night, before a rooster crows, you will deny Me three times."
35Peter said to Him, "Even if I have to die with You, I will not deny You." All the disciples said the same thing too." [Mat 26:33-35]
So much for "certainty."
Peter was a sheep who wandered away and got lost. What did God do? He went out and found him and brought him back. Just like He promised He would. Works every time, unto this day. :)