Four reasons i can think of:

1. It´s 70 years old technology so its harder to acquire than 60s+ technology.
2. The Katyusha rockets had a very limited range of about 5-6km which isn´t nearly enough to hit any major urban areas in Israel
3. Israel usually reacts more or less prportionally to the severity of the attack. A overwhelming barrage would trigger a pretty hefty response from Israel which would crippel the Hizbollah considerably. Instead of doing that its probably better to send a few rockets over the border every now and then to remind the israelis that the Hizbollah is still active without triggering a major response.
4. The effectivness of mass barrages is very limited as this barrage in 2018 shows, why waste a lot of rockets on a single shitty attack if you can use a lot of rockets for a lot of shitty attacks that at least keep the israelis on guard at all times.