Because there are a lot of things we don't like about the current contract. Sometimes it takes many months, even a year, for a first contract to be negotiated. If no agreement is reached, the Union's would either walk away or call a strike. If you went on strike, you would not receive any wages or benefits, and you may not get unemployment compensation. The Company would be permitted to continue business by hiring permanent replacements for any employee on strike.
Strikes are particularly more common when negotiating a first contract.
Unions often call for strikes when they do not get what they want during negotiations. The Company has a responsibility to perform the work it has contracted for. The union says that the Company can't permanently replace employees if we go out on strike. If the union calls you out on strike over money or other benefits, you are an economic striker and can be permanently replaced on your job. You may have to strike whether you want to or not.