The things that make them 'rather obvious scams' to you may not indicate 'rather obvious scam' to someone in a different situation, regardless of any 'mental health issues'. The fact you can't understand that is a character flaw on your part, not theirs.
Things that mark it as 'obvious scam':
They called you instead of sending a letter, etc. - Maybe the person already has debt problems and is used to getting collection calls demanding payment. Whats one more caller? Maybe the person actually HAS tax problems and has received a letter, but has not taken any action.
Indian caller who barely speaks english - 25 years ago if you called customer service for any company you spoke to someone in the US who could speak english. Then companies started contracting out those call centers to India. There was no great announcement that this was happening, but when you called the same number you always called you were now talking to someone in India. Everyone knows their are government contractors. Is it entirely beyond reason that the government would do the same as those companies did? No.
Method of payment - 20 years ago it would have been unthinkable to pay McDonalds or a vending machine with a credit card, now it is commonplace. A very short time ago it would be unthinkable that you just tap your phone and a payment is made, now it is commonplace. It used to be if you wanted a CVS gift card you went to CVS, now you can buy a CVS card at Lowes. Maybe the method of payment requested seems unusual or odd, but is it really any more odd than buying a Coke with a phone?
In short, the more changes you have witnessed in your lifetime the less 'odd' any new changes seem. Things that to YOU may be giant red flags are just another change in an endless stream of changes.
And even if they DO think something is odd they may pay anyway. Most older people are risk averse, and the things they value most are their home and their independence. Threaten to take either of those away and you have triggered a very strong fear response. The risk of losing a little money to a scam pales in comparison to losing their home or independence, so they pay.
So why don't they ask for help if they suspect a scam? Because of assholes like you that think that is a sign they need to be 'taken care of and protected', thereby losing their independence.