Posts saying things like, "Just put them down and let them sleep" are obviously written by people who haven't had awkward sleepers. It isn't always that easy, there isn't always something that you can do right.
I have 2 girls, DD1 is now 2.5, DD2 7 months. They are chalk and cheese when it comes to sleep, and have been right from the beginning. DD1 wouldn't sleep in the day, except on someone, usually me. I got a sling which helped. She wouldn't settle herself and didn't sleep in her cot in the day. I remember spending hours trying to get her to sleep in her cot, I tried different techniques, etc. She had 2 feeds a night until around 10 months when a combination of things, including controlled crying, clicked together and she started sleeping through.
DD2 slept really well right from the beginning. In fact, her first night, in the hospital, she slept 6 hours and a midwife told me off for letting her sleep so long. At about 4 or 5 weeks, she started sleeping 8 hours a night most nights. She settles in her cot by herself, sometimes without even a murmur.
I knew DD1 was a bad sleeper, but didn't realise quite how bad until I had DD2, at one point I wondered if she was sleeping too much, talked to a few friends and family, to find out that it was normal for a baby to sleep that much!
I haven't done anything radically different with these 2, I didn't suddenly work out the answers with DD2, they are just different children.
Yes, some parents let their DCs get into bad habits, but it doesn't mean that all sleep problems are down to the parents.
Some babies are good sleepers, some are not and have to be sleep trained at some point, or it can continue. Some parents don't realise they can even do sleep training, and let things carry on because they don't know what else to do. Sometimes even sleep training fails too.
If you have a good sleeper, it isn't necessarily because you've done everything right. If you have a bad sleeper, it doesn't mean you've done anything wrong.