Tempering seems to require some active effort on the part of the Primal. More over, its not something a Primal can just continuously do without a break. The Company of Heroes strategy was based around this.

They divided into three waves. The first wave would be the sacrificial one, fighting till they were tempered by the Primal. The second wave would then engage the first one and keep it and the other primal followers occupied while the third wave would flank and engage the primal directly. This is how they successfully beat two primals, with admittedly a very high loss of life.

I suspect tempering requires Aether and so Primals don't do it wastefully. In a lot of cases they don't have to. Their beast tribes worship them without the need for tempering.

Curiously with 3.4, I get a feeling the WoD have figured out some way to turn the Primals on the beast tribes that summoned them. We see Titan killing Kobalds, and in the scene with the WoD we see burned ground and a dead Amal'jaa. It would certainly be potentially an effective way to get the beast tribes to stop summoning their primals. That though seems strange to me. Why would the Ascians seek to stop primal summonings now? Perhaps how that Hydaelyn is so weak, further primal summonings might start undermining their own goals? Weaken the host rather than killing it?