Why people ask it
Because they have usually only encountered one Christian position on origins, and it was presented as all-or-nothing.
Short answer
Evolution is a proposed mechanism for how life diversified. A mechanism does not answer why there is a universe with laws that permit any mechanism, and Christians hold a range of views on the biology itself.
Deeper answer
The critical distinction is between evolution as biology and evolutionary naturalism as philosophy. The first is a scientific proposal about how life changed over time. The second adds "and therefore there is no purpose or designer," which is not a laboratory result. It is a philosophical rider attached to one.
Christians genuinely disagree about the biology. Some accept common descent fully as the process God used, some accept parts of it, some reject it. You do not need to settle that before you can have this conversation, and pretending there is one Christian position misrepresents the church.
Notice also what evolution does not address. It operates on existing self-replicating life and does not explain how the first such system arose. And no mechanism explains why there is a lawful universe for mechanisms to operate in.
Common mistake
Arguing technical biology you have not studied. Stay on the philosophical distinction, which is where the actual disagreement lives.
Conversation-ready response
""I want to separate two things: evolution as biology, and the claim that it rules out purpose. The first is science. The second is philosophy. Which one are we discussing?""
Source notes coming soon.