There are many reasons to hide your affiliate links but first most important reason that affiliate links are very long and look ugly. When users mouse hover over your hypertext so they will see that ugly links.
Or if you use short url service so people don’t know that on which site this short url take them and mostly peoples don’t trust short urls and don’t want to wait on short url redirecting page to see many ads.
Second reason is that some bad peoples copy your link then remove affiliate id and buy product directly so you do not get any commission and then there is no reason for you to run your site.
Third reason is that people don’t like blogs which over use affiliate links and they know that you will be paid for these reviews.
Forth reason that google do not give high ranking to sites which have many affiliate links.