Multiple casinos & sportsbooks have multiple promotions and are partners of different athletes. For example, Chips has its own team of UFC and other MMA organizations, so they had exclusive deals for those who would place a bet on their partner fighters.
I assume that true gambler and sportsbook user will be registered on multiple websites, so they'll definitely check odds on each of them. There is a big difference between 1.6 odd and 1.75 odd when you place money, especially a huge amount of money.
Yeah, we have discussed about this if you read what we have posted on this thread. I will lock this thread right now to avoid people not to further repeating what other people have posted.
For those, who aren't active gamblers and sportsbook users and only place a bet once in a while, I assume that registering on different casinos and going through KYC is more headache and doesn't worth for them to check for odds on different websites.
I have many gambling sites that I registered on because of their presence on this forum, they are not KYC casinos. A good example is Metawin.