You can't earn bitcoins without investment. Invesment not only mean money. You can invest your time and effort to earn bitcoins.
seems difficult to earn bitcoin with investment as the value it is high but if you can give your enough time and work hard with patience than hope for the best , you can get it.
I dont agree with you, because there are lot of sites which give you free bitcoins without any investment. You will have to complete some task and then you will get your share in shape of free bitcoins, because many companies are now paying only in bitcoins. And if you want to invest in bitcoin then there is no need to work for others. Just invest in bitcoin and wait a little longer and you will earn because of the rising price of bitcoin.