As has been pointed out, when it comes to companies like WWE and TNA, an "off season" would cost them a lot of revenue. Pro wrestling has long been a staple of cable television and one reason why is because, compared to most other original programming, the production cost is quite cheap. Right now, USA pays somewhere in the neighborhood of $1 million for each episode of Raw. To someone looking in on the outside, that's a whole lot but it really isn't when you think about things. Each cast member of the CBS sitcom The Big Bang Theory makes $350,000 per episode. The main cast features 7 actors & actresses, so that comes to $2,450,000 for just those actors for each new episode. That's not factoring in the production costs for the camera crew, the writers of the show, the director, producer, etc. In some very popular shows, it's not uncommon for the stars to make in excess of $1 million per episode.
Comparatively speaking, pro wrestling has always been something that's drawn significant viewers while costing a fraction of what many other programs cost. Pro wrestling television shows bring new content to the networks they air on every week as opposed to maybe 22-25 weeks a year for more traditional dramas & sitcoms on broadcast networks like ABC, CBS, Fox & NBC. For most reality shows, each season tends to last somewhere in the neighborhood of about 10-12 episodes, whether they air on broadcast or cable networks. The relatively low cost plus new content each week is something that networks like and want.
As it pertains to WWE & TNA, they're generally the two most consistent draws for the networks they air on. Factoring in DVR viewership for Raw, it tends to draw in the 4.5 - 4.7 million viewership range on average; making it the #1 show on USA. SmackDown! over on Syfy has been the biggest draw for the network since it debuted, which will be 3years on October 1st. Again, with DVR, SmackDown! generally draws somewhere close to 3 million, sometimes more. For a cable network, these are great numbers but would be considered pretty lousy for one of the four broadcast networks. For the past month or so, with DVR viewership, Impact Wrestling draws somewhere in the 1.4 - 1.45 million. While not exactly setting the world on fire, they're good numbers compared to most other programming on Spike. Otherwise, I think Impact Wrestling would have been cancelled a long time ago, especially after the Monday Night Experiment.
Plus, there's also revenue from domestic & international house shows, revenue from ppv buys, various merchandise such t-shirts, hats, posters & pictures, DVD sales, etc. In the grand scheme of things, pro wrestling is also something of a niche market. It isn't something that's ingrained into the public consciousness the same way pro football or basketball is. If WWE took 3 or 4 months off, there's no real guarantee that viewers would turn back onto Raw on Mondays or SD! on Fridays. There's no real guarantee that 5,000+ people will start coming back to the house shows like they regularly do.
For wrestling companies on the level of WWE and TNA, an off season could mean millions of dollars down the tubes. Pro wrestlers who want to be able to make the big money must realize that there has to be a trade off. That's always been a known & accepted fact no matter if the year is 2013 or 1973 and that trade off is that you have to work and work hard, even if you don't really feel like it sometimes.