Why? The modus operandi of the Jedi is taking all the fore sensitives they can find when they are babies, the number of force sensitives will stay the same, but there will be many who will not want to join the Jedi, in the same way that there is a fairly small amount of people who will want to join any other religious order that makes you renounce your family and friends, and live by the maxims of Poverty, Chastity, and Obedience.
Especially when there are other places that can teach you how to use the Force and they don´t ask you to renounce to your previous life to join...
Yeah, but the Jedi are a
very well known and respected organisation even if their numbers are dwindling at this time, it took the empire to make people forget about them.
Before then they were heroes legendary knights known from heights of Corusant to even the further reaches of the outer rim.
I'd wager there's plenty of Jedi who've run into older force sensitives who'd leap at a chance to join the great order be a legendary Jedi (I know I might) and its not like they live in poverty, hell I don't even believe there's anything specifically requiring chastity either. As for obedience sure, but I have to be obedient to my teachers ect. and if I'm a Jedi I get to be a hero. Pretty sweet deal.
Instead they get turned away because they're too old despite their hero worship.
Even if there are other options because frankly those other groups like the Blackwatch Makutai ect. they're not very well known...like at all.
Hell if they really don't wanna budge on this issue just give the older sensitives basic training and a promise to help Jedi that need it.
Also your wrong about there not being enough force users, there are plenty and the number is presumably growing with the galactic population, the main issue is that there simply are not enough Jedi to look for em and do their main job.
And this problem is only going to get worse if they remain in the core, too many people drowns out force signatures. There maybe some midiclorian test though I dunno.