Why do I keep getting denied for credit cards?

I have checked eligibility for credit cards from both halifax and santander and have been denied by both and I don't understand why?

I thought that companies gave credit cards and loans to younger people like me (18m) as many people end up building lots of interest?

Anyway, I'm employed and earn around £28,080 a year before tax (I've just had a pay-rise to this) and I have never once even been close to £0 in my bank as I don't spend much. My only real expense is my car insurance (around £177/month) and the rest of my earnings go into one of 3 savings accounts: 1. I can only put £250 in per month but has an interest rate of 5.5%. 2. I put the rest of my money in this one pretty much. 3. Recently opened a Lifetime ISA which I'll be paying £350 into a week. Until it gets to April where I will be able to fill it to the £4,000 and pay less weekly over a while.