Usually, a proxy won’t live for long. It’s because too many people are using it. When you use a proxy, you are using the resource on the proxy server such as its CPU, memory, and bandwidth which are all limited.
Most working proxies on the Internet are public proxy which is open to everyone. Because of the lack of access control, the working proxy will die if the number of users exceeds the capability of the server. If a working proxy can be found easily on the Internet, it usually lives very shortly.