Why do they never say anything?

Guys assess different levels of reward vs risk. Some guys (the shy ones) will only approach you if the reward vs risk ratio is extremely high. Bolder ones will approach when it's much less.

Your physical beauty, your projected personality, your own interest in the guy etc. are all forms of "reward." The possibility of him being rejected, you being the crush of his friend, you having a boyfriend, a whole bunch of guys floating around you, etc. are all "risks."

If you want more guys to approach you, increase the reward vs risk ratio. Careful, however, about falling into the attention whore trap--easily getting guys to be attracted to you, but very difficult to get them to stick around (or remain faithful) long term.

Maybe you're not projecting yourself as "approachable"? Whenever you see them looking at you, smile at them, maybe wink a bit. Try to communicate through body language that you are open to meeting them, and then maybe you'll get better results.