The reason for most of those sites having similar look is the use of the same theme again and again. Someone who makes a new casino often uses pre-made themes for the casino and they just pay some amount to get those themes instead of making the theme on their own by hiring developers.

Most of the new casinos tend to invest some money on their marketing because they want to have as many players on their sites as possible and that they can get only with the help of proper marketing. The marketing allow them to gain some players and they basically make the money back when a few of those new players losses some bets.

Once they get enough money then they start changing the theme and continue making new and better features available for the players. All sites need initial players to get some funds and that's why they offer very high bonuses to attract players attention. The good ones share details in FAQS while the shady ones just trap the players but they somehow need some investment to grow and the new players seem to be the best investors for them.