Because it means they will have to recruit more (flexi ?) crew, which will mean additional cost, reduction in crewing flexibility in relation to line training new crew, loss of experience levels to bridge the gap when considering suitable candidates for upgrade etc. - list could go on am sure.
Why not offer decent terms and conditions from the start ? retain employee loyalty, save some costs long-term, have crew that can see a career progression path, a happy workforce will reap far better rewards in the bigger picture than the short-sighted vision EZY and others seem to nurture. You tend not to hear huge amounts ordinarily about Monarch because in the main i have a very strong suspicion the crews are very happy and motivated.