It sounds like you may be overthinking it & new friendships or acquaintances take time and start gently on the whole. If you don't want to feel so isolated maybe invite one of your DC's friend on a playdate - either to yours or the park or a play area after school (some parents of reception kids want to come along, others are fine not to, either way there is an opportunity to chat at drop off & then next time you see them make eye contact and say hello etc).
There always seems this perception on threads like this that everyone else knows each other - but even if that is the case (and it is unlikely) why would that stop them meeting new people? I've been a 'new' parent twice at school gates (new to the area) and met lovely people, some might have a lifetime history with each other, some might have just met, but it is irrelevant.
However if you stand there looking at the floor it's harder for people to get to know you - and if, like previous posters have suggested you stood there with headphones on I would presume you didn't want to integrate or make small talk/were a bit rude (but also if that's the way you feel then fine, but it does send the message that you aren't at all interested and are fine not being part of things).
Lots of people think 'why bother' etc, I quite like being at least casual friends with the parents of my children's friends as it makes it easier as they grow up, start having sleepovers, go into town together, get given lifts by friends parents etc and become more independent. Also if you were late one day or your car broke down or some emergency came up having friends who you are comfortable with who can look after your child at short notice (and vice versa) who are already stood there at the school gates can be really handy.
There were one set of parents at DS1's school who were really standoffish throughout the whole of primary school and I didn't let my child stay at their house as I wasn't comfortable with it or them. It pays to be at the very least polite and nice IMO.