Most of the time, we lose our assets either to hacker, scammers or simply losing excess to our private storage, this outcome can be traced to lack of follow up with the market and a reflection of the level of disconnection between the investor who just follow the trend of buying Bitcoin and forgetting about it as you stated.
For ultimate goal one need to be actively involved with the market, to know what exactly is happening with the esset market and being able to make decisions that can help safe some situation at times, most especially when one is holding high volitile cryptocurrency assets.
I think that most of the time we do not lose money because of hackers or scammers but because of unsuccessful investments or if someone is doing trading then also because of trading. For example if some beginner decides to invest not in Bitcoin but in an altcoin it does not matter which one I see that all altcoins are performing very badly right now so most likely they will lose money and lose quite a lot. So I believe we lose money most often because of bad investments in crypto.