Why not? In what way is the evidence insufficient?
I'd say the fossil record. We don't have a complete and comprehensive fossil record that shows every species that has existed as well as the transitional forms.
Ofcourse, the more fossils we find, the fuller the picture. And the fuller the picture, the better macroevolution looks.
However, creationists jump on the fact that there are gaps in the fossil records, as well as the difficulty in proving macroevolution can occur in a lab, to say that maybe/probably microevolution occurs but macroevolution doesn't.
The trends dictate otherwise, but since creationists think they already have all the proof they need (Genesis), they simply say it is up to the "atheist evolutionists" to come up with more substantial proof.
Nevermind the fact that we haven't found evidence of a man with no belly button, a garden with a tree protected by a flaming sword, a snake that talks, or any evidence that the universe was created in the order that Genesis depicts, whichever of the two versions you look at.
If you really think about it, creationists that really understand evolution yet question the prospect of macroevolution occuring are either deceptive or plain disingenious. Their hearts and motivation might be in the right place, but the tactics, I consider, are unChristian.
In a way, it shows how science is better than religion in the terms of honesty.
For a religion, the assumptions are made and never questioned. Then, the criteria for truth is limited to only whatever evidence can be used to fulfill those assumptions, which if looked at critically, is a form of deception. Any person who questions the assumptions or the evidence is considered a fool for not recognizing truth according to the preconceived dictates of that religion.
For science, assumptions are made based on previously observed and validated experiences. The criteria for truth is simply that which is consistently observed under controlled conditions. The evidence can either fulfill or call into question previous assumptions, and with enough testing, new truths are discovered and old assumptions are improved upon. Any observation or assumption made in science is open to scrutiny and testing if a person questions the evidence. A person is only a fool if they knowingly manipulate the evidence to fulfill some preconceived assumption or desired result, a practice universally recognized as deceptive and treacherous (and often criminal).
If truth is a virtue, science wins by a landslide. Science implicitly demands honesty by anyone who wants to participate. And everyone is encouraged to participate. And even if they don't, they can still reap the benefits.
I ask Christians, if God created the universe, and the laws of this universe are consistent (and so far have been), then God is consistent in his message and dictates. What religion has maintained the kind of consistency that God evidently exhibits through His universe? Has Christianity through the Bible given humanity any sort of consistent depiction of its Creator? Has every insight into the nature of this universe provided by the Bible square with what we now understand to be reality through science? Why do science and Christianity continue to diverge as science progresses forward in understanding God's designed universe while Christianity remains stagnant and unable to verify its most important assumptions?
Why does the number of Biblical style miracles occuring in this world decrease as people's knowledge and understanding of this world increases. And why do people cling to ancient third person accounts of such miracles when they pale in comparision to the sheer brilliance and extraordinary truths science continue to discover everyday?
There is a trend that is all to clear to those who are willing to simply examine it. However, any call to that sort of examination continues to be suppressed by either wide-spread ignorance or concentrated deception.
Ignorance and lies are the foundation for any successful religion. Look no further than its most ferverent adherents to find the evidence.