I play hours daily, mid 30s lvl. I've had one shiny in months. Absurd. I dont get it.
So part of that is RNG / Luck each encounter can be a shiny or not but until you encounter them you dont know
The other part is working to increase that chance - and how much of it is grinding vs just playing as normal.
The easiest example for this i can give you is community day (other than the last one which was broken) - for community day most of the players in the town I'm in go out and walk for those 3 hours and as we go around different clumps / spawn points / stops we bump into each other and compare how we're doing for shinys
In the first hour there is always someone with 5 or 6 and someone with none - as the time goes and you meet people for a second or third time you find the person with 6 only is up to 8 and the person with none had a little clump of them all together and now has 3
Its very very rare for someone to walk the whole time and not get any , and its also rare to see anyone that got more than say 9 or 10
Apart from one guy , who almost runs laps of the town and just shiny checks every thing he can - he only catches shinys and ignores everything else and he must spend those 3 hours at an almost constant run. If you can get a few words out of him as he passes he's normally 3 or 4 times the number of others (and ends up with 20+ most of the time) , however it looks like a lot of work / pain / effort to get them.
So you can just keep going with your RNG chances , or you can max things (like that guy does) or you can go somewhere in between.
If for example during the last week you just spent time focused on one or two things (e.g. finding and doing the Onyx rewards or just clicking on Anoriths) you'll probably check and lot more and increase your chance of getting a shiny at the cost of doing other things.
I probably put around 2 hours daily in game and did not note my Anorith or Lileep levels before the start but both are at the seen around 400 mark now - and for that I have 1 shiny Anoirth that I got on day 2 - and I got my Shiny Creselia today after doing 1 or 2 raids for them a day since it started.