Kevhen, you bring up a good point but your idea only works on the rarest of exceptions and only for a certain level of play. Many people (generally adults) paying for lessons do reach low plateaus and yes they won't show huge improvements even after years. This is contingent on alot of things though. Primarily their playstyle and athletic ability.
A solid commitment to training can make serious improvements to a player who has played poorly for years, this requires a huge effort which many are not willing to do. When I first started teaching I found it really disheartening to be teaching these people with horrible swing styles and horrible bushel basket rackets which only encouraged their terrible swing. To really get them out of the plateau they are in, it would require often a complete breakdown of their swing mechanics and rebuilding. This takes alot of time and effort which most adults are not willing to commit to, nor do they find it fun. People take lessons for all different reasons and surprisingly many do it for a guranteed workout and good hitting partner. Of course they want to play better but having fun and getting a workout is more of the priority to these people then rebuilding proper swing mechanics to get to 4.0+.
Another factor is the instructors. Go to your tennis clubs and look at some of the instructors. You will find many adult instructers are little better then feeders. They give half hearted instruction and do not really try to rebuild a good solid technique but make band aids to a bad swing style. You can't always blame them though due to what the client wants (as stated above).
People have different expectations and needs from tennis and lessons. We view things differently. Let's face it we are all here chatting about the game, equipment, mechanics, etc on a forum on our off time. We obviously view the game in a more serious and competitive manner then someone who plays with a 3" wide racquet and pays for a lessons once in a while at a expensive country club. Yes it's very true that some people are paying for lessons on a regular basis and not getting much better, but improving your game is al relative. While we may compare ourselves to higher standards, someone taking lessons might just be happy they learned to put the ball in the court better after 12 sessions of lessons.
As for getting free tips here and there. Not all players are friendly, knowledgeable, nor are the environments always condusive to instruction. Even if you are getting free advice most of it is coming from unqualified individuals, people who do not follow your style and progress, with the inability to properly work out changes and improvements with fed balls. I don't care who you are. Fed balls with proper instruction will ALWAYS help you learn mechanics better then trying to figure it out during a set or even rallying. Advanced understanding, yes you can improve certain aspects while hitting, but many fundamental things are learned faster, and often more accurately without bad habits by proper instruction and drilling. If by some miracle you get some knowledgable people who are willing to give you time here and there, more power to you, but the majority of us do not have that luxury.
Your girlfriend is lucky to be have access to free instruction and help. I know the ladies are gonna hate this one but... If I had boobs and and tennis skirt, I guarantee you I would get more free tennis tips
I am just kidding, I mean no offense.
I will even go as far as to state that I am one of those exceptions as well. (no I dont have boobs or wear skirts) Good players and luckily good coaches saw something in me and decided to take me under thier wing. I never had to pay for 1 lesson in my life. It often started by them seeing me hit and they would tell me a few things. I picked them up very fast and it piqued their interest. Soon I was helping them pickup balls/feed lessons and in between they taught me and also drilled me. Didn't take long for me to really improve my game from a wild and crazy flat hitting 2.5 to about a solid topsin 4.0 player in about a year. After that it was slow going though. Without more hours a day spent on my own drilling and match practices, I wasn't going above 4.0 so easily. I eventually was working out (not playing matches) against Div 1 college/semi pros and even a few pros. That is not to say I was as good as them, not even close but my rallying shots and practice points were good enough to work them out. Still, even though I got it for free, I really didn't since I partly worked with them, and it took several years of intense working and instructing to get there (I also had great coaches one was a NCAA head coach who let me practice with the team 2-3 times a week).
Again these are all rare exceptions and not something people can count on. In your day to day most people do not get very helpful tips or ones they are exercise to get better, at least not in the way drilling and lessons do. Also as I said, people have different needs and expecations from lessons. Don't frown on them if you are getting better then they are without lessons and they are paying $50/hour. We all have different learning curves and ability.
If you never took lessons and are serious about getting significantly better, try asking around to some good players (4.0+) and see who they recommend. Get 3 lessons and see how you feel about it after those 3.