I volunteer for Oxfam - a charity which was hit by a huge scandal about 4 years ago. What went on was horrendous and there is no excuse for it, but a couple of highly dodgy people (not just at Oxfam) have cast a shadow over the whole industry, despite changes being made to make sure nothing like that can ever happen again, and the people involved never having been employed in the the UK.
Oxfam is one of those charities with lots of money in the bank and lots of investments. Because if there is an earthquake or flood or other natural disaster tomorrow they need those reserves to charter aircraft to get all the warehoused tents and water purifiers out to where they are needed, and to hire local ground staff to coordinate the response. With the best will in the world, the logistics of getting aid to an emergency zone can't be managed by Sylvia, the former library assistant who now does the till on a Tuesday afternoon.
Deep down people sneer at charity volunteers (any charity) because they think we make them look bad. We're the ones pitching up every week and sorting through bags of old clothes, or keeping the shop looking nice, or running the PTA disco, or standing in the freezing cold cheering on the Parkrunners. People who are not involved know that deep down, we're doing a good thing. And they are not comfortable knowing that they are not contributing in the same way. So we are grabby, self-serving, pigs with snouts in the trough, in it for ourselves, woke - whatever other insult they'd like to throw.
Of COURSE volunteers enjoy what they are doing and get something out of it. Who is going to volunteer otherwise? Some people want to give something back after years working, or do a sponsored walk for a cancer centre which treated their mum. What other people get from volunteering might be the sense of community, or work experience, or just spending time with people they aren't related to. I love my weekly shifts in our shop, the ladies are fabulous, I have learned so much from them about ceramics, jewellery and linens. We have a chat and a coffee and make lots of money for a charity we all think is worth donating time for. And buy lots from the shop too.
And finally - all the nonsense about charity paying super high salaries. There's a website called "Charity Jobs" or you can look on charity websites for vacancies. Cancer Research, for example, are advertising for a store manager. 37.5 hours a week. Salary between £19.3k and £24.3k - to get top of the scale you'd have to have loads of experience. So say £21k for someone with a few years experience. For that salary you would be the only salaried staff member. You'd be doing everything from banking to training new staff to H&S compliance to working on the till. No paid deputy supporting you, probably. Trying to recruit enough volunteers to keep your shop open, cover costs and hit targets. All the responsibility of being a keyholder. No overtime. Statutory minimum holiday, pension, sick pay etc. Or, you could go and work for somewhere like Next or M&S where there would be a whole management team, lots of support and cover when you're on holiday, you get 25% staff discount, money towards your uniform, incentives, discounts with other brands etc etc etc.
And as for the old "I read about a charity which only donates 3% to the cause" - did ye, aye? I have read a lot of stuff. Lots of it "fake news". Have we had anyone say that the only reason people volunteer in charity shops is to steal all the stuff yet? 
((Rant Over))