Why do people call themselves experienced lifters after just 1 year of training.?

Because they don't have a long-term understanding of weight training and the associated lifestyle. I wouldn't break things down so much by time as by relative strength and physical condition. Just because you learn things every day doesn't necessarily mean you're still a beginner. I consider myself an intermediate lifter on these standards:

-The easy gains most beginners get for a year or two have long since tapered off, but I'm still able to make slow/steady progress when I pulse up my training or diet.

-I've achieved a 1.75x bw benchpress, a 2x bw olympic front squat, and a 2.5x bw deadlift.

-I believe my deltoid complex is still in it's beginning stage due to my recent isolation of it. I've just recently been able to complete a strict standing military press with my bodyweight. I think 1.25-1.5x bw would be an intermediate number.

I suppose you could classify an intermediate lifter (to me) with these terms. I use relative strength ratios but have included straight pounds lifted for those who are naturally too big to have double-to-triple bw lifts:

Military press: 1.25xbw -OR- 225lbs.

Benchpress: 1.75xbw -OR- 315lbs.

Squat: 2xbw -OR- 405lbs.

Deadlift: 2.5xbw -OR- 500lbs.

These obviously should be done with no assistance from a spotter. The squats are ***-to-grass and there should be no use of momentum in the presses. I feel these are numbers that 99% of the general population could not achieve without significant effort and training.

Just my .02

I would think because they think they know it all.