This is not true at all. Baby's, even at a few months old, run the gamut from excited to meet new people to shy and withdrawn around strangers.
I have a friend whose son, at 6 months old, was well socialized but completely uneager to interact with people he didn't know.
And I've met plenty of toddlers who hide behind their parents and don't come out, and plenty of toddlers who run up to everyone they come across and say "hi".
Shyness and/or fear around strange adults is a healthy biological response. Even children raised in isolation and who show immediate fear of other new children are still quick to make friends when introduced to pre-school (a terrifying experience altogether).
Children who cannot be socialized effectively by around age four (5% of boys, a much smaller % of girls) usually end up punished explicitly by society in their later youth and early adulthood.
You're enabling your own prescription by making the problem easier, viz. reducing the problem of anxiety to "not feeling confident about the appropriate actions to take", which, I might add, is also a redundant claim since its taking an effect of anxiety and elevating it to the cause.
Anxiety, for most people, is not unsolvable but is just tedious because it often comes from multiple sources. I'm diagnosed bipolar, ADHD, OCD, you name it... I am no stranger to anxiety and the physical aspect of it which cannot be dealt with "logically".
But lack of confidence, in one's life, one's friends, one's job, one's ability, etc... this is a major source of anxiety in the modern world and is very treatable.
In a way, it boils down to “not feeling confident about words I say”, but the problem is I seem to overvalue bad outcomes, or at least think about them deeply rather than assume the very likely path will happen. Some of it is lack of a quick-witted theory of mind mapped to others (asd, add, who knows) and some is past trauma where many people I knew growing up did a lot of hurtful things. The weights on how must one feels they can trust the average person are very slow to change - and the what-ifs seem to come on nearly automatically.
I also have had problems with being too trusting in the past and lately I've been having social anxiety I thought I'd solved return because I've been trying to make conscious changes to my social behavior and it leaves me feeling lost sometimes when I have to ignore my instinctual response and then can't think of anything else to say.
It may comfort you to know that you aren't alone in feeling you need to reevaluate how much you trust those around you.
Social research in the last 3 decades has revealed that, in America, and thus probably other modernized countries, as late as the 80's, people when asked if they could trust most other people generally said that they felt they could. This is not the case anymore. Now, the average person will proclaim that most people are not not trustworthy. People are reporting less friends than ever. I personally have a quite small group of friends, after I carved out most of them after having 4-6k stolen from me in the last couple years from various people I once would consider friends.
Isolation and mistrust have been bred into our society in the last quarter-century, and it has had disastrous implications on our ability to open up to others. We are starting to feel the effects of this as, on a global scale, the isolation people are experiencing causes them to lose empathy and adopt zealous and hyperjudgemental attitudes.
Why has this been happening? What's the motivation? Well that's the homework assignment. ;)
> Even if involuntary they 'trained' people's social skills and withered away at their acquired/innate social anxiety.
Humans don't have a shred of innate social anxiety except perhaps in some odd pathological cases. Any parent who has dealt with a toddler that has yet to learn to talk will tell you that much. Toddlers who can't speak yet try very hard to interact with their parents and whoever else they meet. They've a fascinating ability to get themselves understood by their parents.
Science will tell you that much too. Humans are basically hardwired to communicate. Intensely. So much so that if you gather a bunch of deaf kids together without supervision they invent their own sign language. [0]
With that being said, kids do learn (or unlearn [1]) things at school. Including how to "socialize" (or not).
[0]: https://www.sciencedaily.com/releases/2004/09/040920071439.h...
[1]: https://www.ted.com/talks/ken_robinson_says_schools_kill_cre...