If I'm understanding correctly, I think the question should be "Why sites loads websites gradually?" .
See to answer your strict question, browsers do whatever you (where you is the developer, site owner, etc) tell them to do. There's no magic, if you tell the browser "do A, then B, then C" that's exactly what the browser will do.
However, if the question is "Why sites loads websites gradually?", then there are many reasons.
As explained before, the browser will follow instructions, and also the order of these instructions. This allows the developer to control how a site loads, and also causes a lot of problems to people who doesn't know the caveats of site rendering.
Just think of this: a browser loads resources: scripts, images, text, connects to a DB, loads external resources, and so on. Generally, you measure this in milliseconds, with most professional sites loading under 2 seconds.
If properly done, you should never see what you say. These load times are smaller than most people can perceive, so a professional site should load "completely" before you notice.
However, there are situations where this doesn't happen, including (but not limited to):
For the first item, there's no much to be done, but the other 3 can be easily fixed by the developer or site owner. Whether they're fixed or not is a whole different story.
OK, but I have a mega site, what should I do?
There's an obvious case where sites will load slowly: sites with lots and lots of content. This was specially true in sites with Flash , and now you can see it in some sites loading videos or heavy files. In this case, some sites with no optimization (or optimized to the limit, and yet slow), choose to show a loader page (or in some cases, a static splash page).
Quite honestly, in modern development this is rarely needed. And if so, there's a chance the problem comes from another reason (such as poor UX, unneeded content, bloated external resources and such). They're basically yelling at you: "hey, I have no idea how to optimize my site, but wait all this time because my stuff is so amazingly amazing it will blow your socks off, promise!" and then a loader that might (or might not) end some time. Hopefully, it will end before the user runs away. Hopefully. And here comes something you might have seen many times before: the dreaded never ending loader which is in place because the page is very slow. And the page is very slow because... a script is blocking everything. Fixing that would make that loader unnecessary!
- Assuming you mean sites rather than browsers: mostly poor decisions or lack of ability
- Assuming you really mean browsers: because that's how browsers work, they have to load things in some way after all!