most web pages already have lots of 'animated entrances' in the form of the entire page hopping around all over the place as elements are loaded via client side rendering, flashes of unstyled text, etc. for example, here's what this css-tricks webpage looks like when the page loads [0]. any CSS page loading animation you add to this page would have to 'compete' with the unsightly effects of the page trying to go through several steps to render itself.
i think it's always such a relief when i click on a link to any site, and the page loads, and there's no animated anything whatsoever.