Well, there are a few reasons. Obviously, I'm speaking as a liberal.
One, people don't all have the same values. Therefore, we don't all have the same interests.
Second of all, since this question is nearly always asked my wealthy leftists, just think about how how many wealthy leftists have been celebrated by calling for higher taxes on themselves? So, wealthy leftists voting against their interests because they're supporting their values isn't mystifying but conservatives doing the same mystifies you?
Third of all, you are aware that we don't agree on the effects that any policy has. No, money and fame are not measures of intellect. Books aren't all that expensive. Your brain was given to you at no cost.
Really though, most of all, since this is always as asked as a rhetorical question, it translates into "We want to help you. Why won't you let us help you?" Of course, that's putting aside William Graham Sumner's The Forgotten Man in which he explains that that kind of 'help' is really people saying, "You're suffering. I get this other dude to help you."
Have none of you who have asked that question ever thought that people don't want your help? At least, they don't want your help in that way? Moreover, has it ever occurred to you that there are people out there who feel insulted, humiliated, like absolute failures to ask for help and would rather suffer through on what they have? Maybe, people in need would rather take the humble and honest and personal charity of a friend providing a couch than take section 8 housing from a bloated bureaucracy financed by force from people who don't know us?
Surprise surprise, sometimes people's values circumvent our interests. For a lot of people, our values and our interests are regularly the same thing.
A problem is that we don't take diversity seriously enough. If we truly are truly diverse, then our capabilities, limitations, desires, and goals are diverse also. Consequently, what someone thinks is good for me is not necessarily what I think is good for me. And if we go a step further and take equality seriously, then that someone should leave me alone to pursue what I think is good for me.