i) Because Mark, which is commonly thought to be the earliest canonical Gospel, doesn't have an account of the risen Christ, some unbelievers think the Resurrection narratives in Matthew and Luke represent a legendary embellishment of Mark.
ii) A few scholars surmise that the original ending of Mark was lost. If so, then the original ending presumably reported the Resurrection. Of course, that theory can't be proven or disproven.
iii) Textual criticism aside, Mark contains predictions of the Resurrection (Mk 8:31; 9:9,31; 10:34). He records the empty tomb, along with the angelic confirmation of the Resurrection, and prediction of a post-resurrection appearance in Galilee (16:4-7). Therefore, the Resurrection narratives in Matthew and Luke aren't simply tacked onto Mark, in spite of Mark. Mark itself had that expectation, as well as a terse fulfillment.
iv) If our extant MSS of Mark did contain a Resurrection appearances (or appearances), then unbelievers would discount the Synoptical parallels in Matthew and Luke. They'd say Matthew and Luke simply copied their Resurrection narratives from Mark. They'd say Matthew and Luke simply got their information from Mark.
As it stands, the absence of a Resurrection appearance in Mark means that Matthew and Luke provide independent, multiple-attestation. Absent a Markan precedent, that's what we're left with. They didn't get it from Mark, and they didn't get it from each other. So Matthew and Luke each had his own, separate sources of information on that score.
v) And, of course, the Resurrection of Jesus is one of the most widely-attested events in the NT. It isn't confined to the Gospels.