Can a hardwired security system be installed after drywall is in place?

I want to have a hard wired security system installed in a house that was built in 1993. I received an estimate from an alarm company with almost all wireless components. When I replied that I did not want the components to be visible on the windows they said you cannot install a hardwired system when the drywall is already in place. I think they are either ill informed or are being lazy.