If a user wants to keep a chatroom active forever, why does SE have a problem? At least, there should be an option to opt for saving chatroom from automatic freeze or deletion.
Who said they don't?
From the Chat FAQ:
Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.
Rooms not worth retaining which are inactive for 7 days will be deleted. Rooms worth retaining which are inactive for 14 days will be frozen. Frozen rooms do not allow any new messages to be sent, and are not shown in the default room list to prevent cluttering the rooms interface.
Honestly, Stack Exchange is not really a hosting service. Yeah, they'll preserve your content, but they're also not really obligated to do so either. If people aren't talking in a chatroom, it's a waste of resources for them.
Addressing your comment:
Let's say, I have created a chatroom to discuss a TV show with spoilers. Users will use it when a season is online. Why would you expect them to use when the show isn't running? When next season arrives, voila... that chatroom is no more.
That sounds like off-topic discussion. To give the benefit of the doubt, let's say that it was on-topic. If a chatroom goes out, you can always flag for a moderator to unfreeze it, but you better have good reason too. There's plenty of chatrooms that are also fairly active network-wide that you can participate in. Why not use one of those as well?