Why does Weyland have to hide that he was on the ship?

I got the impression Weyland did not want the scientists (especially the two head scientists) to realize his selfish reason for wanting to contact the engineers. He kept them under the impression that he was funding the mission so that mankind could have the knowledge, even faking that he was already dead, the ultimate way to get credibility.

If he had been honest and said "I want the engineers to help me live forever" the scientists might not have gone along as he wanted them to. (For example, David suggests Weyland wanted Elizabeth Shaw there as a good-luck charm).

Remember the scene where Vickers tells the two scientists that they are not to make contact with the engineers? This is the first hint that Weyland (in the hologram) was lying. Vickers knew Weyland was alive, and knew his true goal, and did not want the scientists to interfere with that goal.

I would suggest that a man of his wealth, extreme age and extreme desire for immortality would live by standards of caution that few people could imagine or afford. (I know of one real-life billionaire (and there are probably many such men of wealth who share his stance) who is very, very careful about his health and safety and funds medical research.)

So there would be no upside in letting multiple strangers know that he was aboard a ship in hibernation for two years -- he was very vulnerable.

Having said that, just traveling so far from Earth was risky -- why not stay in hibernation on Earth until David returns with the "cure?" Perhaps Weyland felt that only someone with his genius and charisma could effectively negotiate with the aliens.

But I also find it implausible that the sort of technology which produced David and/or hibernation would not be adaptable to extending human life. Or simply that a civilization that had these techs not to mention FTL spacecraft would not also be extremely advanced in the biological sciences. Weyland was old, probably well over 100 (maybe this is discussed in a novel or in short films) and he looked it -- no evidence at all that medical technology had significantly extended healthy human life.