"Due to many requests, the submission deadline has been extended" is this real?

Warning: personal opinion.

I view conferences that regularly shift submission deadlines as a little suspect. You're right that this happens in CS conferences, and it soon becomes pointless because everyone expects the extension ("Nobody expects the extension!!"). But your estimate of 80% is rather off. Almost none of the (many) conferences I submit to have deadline extensions for submission of the full paper.

And don't underestimate the level of pleading that goes on. People will always ask for an extension if they have even a smidgen of hope that the pleading will work. It's only when a conference takes a firm stand for many years that people stop asking.

Extensions are common in conferences because many people procrastinate and or otherwise just need a little more time to complete their paper, prompting a heavy demand for extensions. Unless the conference is overwhelmed by submissions (which is very rarely the case), there's usually room for a little bit of flexibility in the submission deadline.

It's not a lie... Organizers tend to anticipate it based on past experiences in previous conferences.

This is outside CS, but in my field, I've only had three conferences extend submissions, and they were all for low numbers of submissions.

• One I know because the day a office mate and I submitted (the last official deadline day) we were abstracts 001 and 002, and we were not early morning people. This one, ironically, is probably the closest to your experience, and arguably was a CS conference, even though I'm not in CS.
• One has a chronic problem with this, because the session they're recruiting for (its a big enough conference that there are multiple sessions each with their own abstract committees) is a lot of work without much payoff.
• One was because the sequester made travel by federal employees in the U.S. impossible, which meant a quarter of the abstracts were suddenly gone. So the "late breaker" deadline got moved way back.

So in my experience its fairly uncommon, and either symptomatic of a larger problem with submissions, or some unforeseen circumstance.