Why wasn't I told in my 30s the hassle of having breast implants later in life?

Many reasons. There is strong motivation to get breast implants and many patients dont want to hear the negatives. There is a good monetary pay out to the surgeon, who might slant the conversation in favor of surgery. If you are having hassles with the implants after a number of years, you should see a breast surgeon as the implants can rupture, encapsulate and get hard, migrate or become unshapely.
True hassles are few. I'm sorry if you didn't get or hear all the information available when you had your surgery. We now know much more, and have better implants and surgical techniques. However, nothing in life is "guaranteed hassle-free" and the vast majority of women who ask for breast implants are happy with them for decades. Problems are few with well-done surgery by experienced abps-certified plastic surgeons.
Great question! It is my sincere hope that you were told in your 30's when you got them that there would be issues. These are not lifetime devices. If you weren't then I apologize, you should have been. Without knowing your age I can't know when they were placed. Talk with a plastic surgeon to discuss all of your current options.
Hard to say... Maybe you didn't hear. Maybe the doctor didn't say. Maybe we didn't know to tell. In spite of all the hassles, most women I see who come in with aging implants for removal are desperate to get new ones put in. Most surgical treatments have future risk. There is a lifetime risk of bowel obstruction after appendectomy of about 1%.
Informed Consent! If you had your surgery by a board certified plastic surgeon, I am sure you were given all the possible complications of the surgey. If you are talking about sagging then that is inevtable as gravity will work. If you are cmplaining about anything specific, I would suggest that you see a board certified plastic surgeon. There is no need to have hassle in life if they are avoidable.
How old are you? Depending on how long ago you had implants, doctors may not have known the long-term results. Other times I find that my patients aren't listening to what I say. Maybe they are nervous but when something happens, occasionally someone says, "you never told me this could happen! " it happened enough I now write down a long list of possibilities and make them sign it before surgery.
Medical Devices. It is unfortunate when any patient feels misinformed. With the present state of the internet, that is currently unimaginable. However, implants are medical devices and as such are subject to complications. Perhaps the best solution would be to remove the implants and not subject yourself to any future surgery. There are no current implants that last forever or function consistently with time.