1) I believe the ideal is NOT individualism in any form. Theologically, there are SO many examples of this concept within Mormonism.
a) "We without them cannot be made perfect (whole, complete, fully developed)." That is the foundational statement, perhaps, that underlies the theological basis for the earthly application.
b) "Neither is the man without the woman, neither the woman without the man, in the Lord."
c) "Salvation" can be described as an individual gift of grace, but "exaltation" always is taught as being comprised of more than an individual (or even individuals). The use of the word "sealing" is important, and it's important to remember that it's not used EVER to talk about sealing an individual to God; rather, it is used to describe sealing people to each other and those people, as a community / group, to God.
b) Zion is a vision of unequal people voluntarily cooperating to remove the excesses of their individual existences in order to create an "equality range" in all ways possible. (That is my own term, made up for this post, but I think it is much more precise and accurate than other ways I've heard people talk about Zion.) In practical terms, that means those at the "top" giving up something of their "topness" to raise those at the bottom out of the limitations of their "bottomness". Zion does not have to be a group of people who are totally equal in all ways, including financially; it can be a group of people who are united in all ways despite practical inequalities - as long as those practical inequalities are not large enough to divide. The first can be achieved, theoretically, in mortality; the second removes the community from mortal constraints and leads to "being lifted up".
(I believe when the story of the City of Enoch is considered as a grand mythological description of the entirety of mortality and the "exaltation" (lifting up) of a community, all kinds of things suddenly come into focus that are amazing to ponder.)
Those who "have not" cannot demand to be lifted up by the rich if they are not involved in the lifting up of those in the community they can serve, despite their poverty. To be Zion, ALL must lift ALL. There are no "free loaders" in Zion - meaning those who could assist others in some valuable way but don't. In theory, all is available to all, so the poor don't "possess exclusively" their "stuff" (including their money) - just like the rich don't possess exclusively theirs.
Therefore, if the ideal is to share all with all in such a way that there are no "needs" (or even "righteous wants") that remain unmet, ALL must be willing to do whatever they can to contribute SOMETHING to the welfare of the community in all possible ways - and it's really, really, really hard (if not impossible) to do that in mortality in an objective way that doesn't include a percent-based, financial contribution. (Otherwise, it would be easy to argue that a doctor or CEO could contribute one hour of her time to equal 20 hours of a janitor's time, for example.) Thus, in Zion, ALL contribute in ALL ways to the benefit of ALL - to varying degrees, based on their ability.
Financially, in mortality, however, in order to avoid having to analyze every situation and come up with a donation that is ideal for each individual (and perpetuate much of what killed the United Order attempt in the 1800's, in my opinion), everyone is assigned a percent of their "income / increase / profit / net worth / whatever" to donate - trusting that those who have excess will make up the difference for those whose contributions place them in a situation where they require assistance from the communal pot.
2) This is a "communal / governmental structure" first and foremost, not an individual-focused structure - and it is much closer to the United Order ideal than most people understand, I believe, when considered in theory and not in how it actually gets implemented and in how most members see it played out publicly. (There is much of this that occurs behind-the-scenes, whenever a Bishop or Branch President approaches a wealthy member and asks for an anonymous donation to help with XYZ situation - but, even then, it doesn't happen as often as the ideal teaches.)
3) So, in summary, we teach individual self-reliance, in theory, as a way to achieve communal self-reliance in practice - because without communal self-reliance individual self-reliance simply is not possible for all.