That tithe concept developed back in an age where there was no such thing as government support for the poor or the sick. Churches (or, if you look back long enough, The Church) provided 100% of what would now be considered assisted living centers for the elderly, food/clothing/firewood banks for the poor, hospitals, care for orphans, care for the differently abled, and a form of public education. Got an incurable disease? The Church would set you up in a convent or abbey, where you could live out the rest of your days in as much comfort as could be arranged.
At that time, people gave 10% of their income or what they produced (either in cash or in goods) and frequently left large bequests for the Church, whether they believed in it or not. It was a simple matter of funding the services they, or their family members, fully expected to use at some point in their lives.
As a system, it worked for the vast majority of the people who participated. Definitely there was corruption and inefficiency, but the Church lasted as a social institution for longer than most nations, chiefly because it provided so much, to so many.
Nowadays, most of the social services previously provided by the Church have been taken over either by federal or state governments, or by private charities. Although some churches do have a homeless shelter or soup kitchen as part of their ministry, the focus of the churches has shifted toward promotion of religion as being the primary (and, in some cases, the only) goal. Not everybody believes that's worth 10% of a person's gross income, especially when about 30% of that income is also being taken by the government to provide all the services the Church used to be on the hook for.