That’s because you’ve shave that hair for 20 years.
Hair that isn’t normally shaved poses a greater risk for ingrown hairs.
To reiterate, in grown hair in your armpit is awful. It will get infected unless you wear tank tops all the time, and an infection near all the nerve endings in your arm put causes for a seriously painful time. It can all be avoided by just trimming without going down to the skin.
If you do go down to the skin on an area you’ve never shaved before, you’d be wise to do so using conditioner on your armpits instead of shaving cream. You’d also be wise to apply aftershave afterwards.
But you’re right. Just as most men don’t get in grown hairs in their face from shaving after the first 3 or 4 shaves, you are fine, given that hair in your armpit is basically expecting to be shaved.