Because strip clubs normalise the objectification of women, seeing them as consumables and whose bodies can be bought for sexual gratification/titillation. If this becomes an acceptable pastime, just a bit of fun, it feeds into a culture where women can be divided into two kinds: the ones that we are allowed to objectify, and our mothers/sisters/wives/daughters, who deserve respect. However, the ones we are "allowed" to objectify are of course worth less, as they are not seen as fully human, as persons, which makes it much easier to treat them badly, should a need or opportunity arise, and easier to deem maltreatment by others acceptable behaviour.
The unequal relationship between the client and the stripper is part of what is gratifying for the client, as it is the woman's job to please him, thereby confirming his authority as a man. Perhaps as a result of this, interviews with clients have shown that men who frequent strip clubs become less sensitive to the needs of women, and less tolerant of women's expectations for equality in the workplace and at home. It feeds into an attitude of entitlement and a culture that make rape myths possible, domestic violence not taken seriously etc etc.
It is in the interest of the strip clubs that they are seen as normal. So they try to get us to fall for the idea that strippers are empowered, and that those being against it are old fashioned, prudish, killjoy, uncool, and sexually repressed. Strippers themselves may claim they feel empowered, because that offers a way of self-justification. Ex-strippers however, often have a different story about how they felt stripping for strangers.