Consider:
1) An expert witness is useless unless what they say makes some sense to non-experts.
2) The cross-examining lawyer will have done large amounts of research and likely talking to other experts, planning what questions to ask.
3) Expert witnesses are supposed to be impartial. They’re there to offer the benefit of their expert understanding; not to actually argue about the case itself.
4) The cross-examining lawyer may not need to challenge the expertise of the witness. The lawyer is interested in the points of the case, and may be able to either get the witness to agree to make somewhat different comments than the lawyer who called the expert had them make. Or if not, they may be able to get them to bring up points the calling lawyer did not ask about, or to equivocate, or to moderate their conclusions or certainty, so that it has less impact on the case.
5) Some experts may not be great communicators. What matters is what the judge and/or jury think the expert has said, and what part is relevant to the case. So, a lawyer may be able to get the expert to say things that will confuse the other listeners, introduce doubt, or otherwise induce human reactions that help they cross-examining lawyer’s case.