Think of what rate of interest a bank makes offering a home loan - it will be quite low, but the risk is quite low as well.
Now think of what rate of interest a bank makes on a credit card - it's quite a bit higher because the risk is greater that they can't get their money back in something goes wrong.
Realistically, a bank is making around 20% pa. on credit card lending - if a 'safe' 100% pa. rate of return was available don't you think that the big players would already be doing it?
If you're a dedicated and well experienced trader, then you can definitely make a 100% gain in a year if you fully make use of all the price swings.
Unless you have any experience with technical analysis, trading might as well be a coin flip. Most people who are "trading" aren't doing anything more than speculating and hoping they're lucky enough to buy lower than it will eventually be. There's no reason to expect that it will be in most cases.
These people you are talking about are more gambling than actually trading. There are a lot of these people. One thing they all have in common, is that they mostly blame Bitcoin for being manipulated if the price start to tank heavily. That's not how a real knowledgeable trader works.