There are 2 possibilities.
Your overview is set up wrong and you can’t see asteroids that are actually there. If you can see asteroids elsewhere then you can rule this out.
Somebody, either player or NPC, already mined out the belts you checked. It is possible to clear an asteroid belt - they respawn after downtime.
Whatever the case, I assure you that there are still asteroids in lowsec, you may just have been looking in the wrong places.
One suggestion, if you want to try lowsec mining, look for ore anomalies (visible in your probe scanner window) rather than belts. Lowsec mining is risky, so it makes sense to focus on the most valuable ores that you can find. These anomalies may have nullsec ores worth more than the standard lowsec stuff, and I don’t think NPCs can mine them out.
While you are out there, pay attention to your surroundings and assume that anyone else in system is trying to find and kill you, unless you know otherwise. In a Venture you can run away pretty easily if someone shows up as long as you don’t let them catch you afk.