Why is Thunderlord's Decree so popular?

I mean, really. Today, someone recommended I play it rather than Deathfire Touch on Veigar. They said "Trust me, it will increase your burst," but I used math to show that DeathFire Touch is demonstrably better; it deals more damage in the optimal combo and in most situations. Tlord does stack once if you land your Event Horizon, yes, but if you miss it, it is unlikely that you will even proc this mastery once, let alone have it provide more damage for you. Now, here's my reasoning. For this example, we'll assume that this particular Veigar has exactly 1000 ap at level 18, so that our calculations are consistent. We will also assume that this champion has 0 bonus attack damage. So, first, let's see how much damage Thunderlord's Decree will do:

(10 x level) + ( .3 x attack damage) + (.1 x ability power) (10 x 18) + (.3 x 0) + (.1 x 1000) (180) + (0) + (100) 280

Hm. Quite formidable, for something that you just proc and don't have to buy an item for. However, consider how much damage DFT:

8 + ( .65 x attack damage ) + ( .25 x ability power ) over 4 seconds 8 + ( .65 x 0 ) + ( .25 x 1000 ) / 4 seconds 8 + (0) + (250) / 4 seconds 258 / 4 seconds 129 / 2 seconds (calculated because Dark Matter counts as an AoE, which makes the proc last only 2 seconds, instead of 4)

Now, you may be thinking, "Del, clearly, DFT does less damage. What gives?" And you'd be correct. However, what you overlook is how often you will be dealing damage from DFT, rather than Tlord's Decree. In fact, let's chart out some optimal scenarios, shall we?

Now, if we take the classic optimized combo, we will assume that it goes as follows, accounting the damage done by both passives:

                                             ThunderLord's Decree damage total               DeathFire Touch's damage total

Land Event Horizon 0 0 Primordial Burst hits 0 258 Dark Matter hits 280 387 Land Baleful Strike 280 645

Now, you may be thinking, "Del, that's not fair! This is only optimized for DFT's Damage output, not Tlord's! If you autoattack, then Tlord will proc more often!" That would be a fair point, but now I have to point out that you would either kill your target or die if you tried to optimize for Tlord with Veigar. To out-damage DFT with Tlord, you need to do the following sequence:

                                             ThunderLord's Decree damage total               DeathFire Touch's damage total

Land Event Horizon 0 0 Auto-Attack 0 0 Primordial Burst hits 280 258 Auto-Attack 280 258 Dark Matter hits 280 387 Auto-Attack 560 387 Land Baleful Strike 560 645 Auto-Attack 560 645 Auto-Attack 840 645

Alternatively, you could Auto-Attack before Event Horizon, but the point still stands. You need to auto way too many times for this combo to be feasible. You may say, "Del, that's also not fair, because Tlord's will do more damage at lower levels, or when you have less AP. It's for winning lane, not scaling into the late game. You can also hit more than one champion with it if they're standing next to each other. Also, it's not too hard to proc if you git gud you Bronze scrublord."

Well, you would be correct. However, Veigar is meant to scale so hard after he's full build, he'll smack you over then head with raw AP, and maybe some spell pen. Now, this doesn't always apply, but it applies to a lot of burst mages. Most of them don't have the attack speed to pull combos like this off. Riven can use Tlord well, some other champions can do it well, but why do I see entire teams with Tlord's, and why is everyone so in love with this mastery? In fact, everyone says that you should be playing this on all burst mages, but that's not always the case. Sure, if you have something that can proc it three times easy in lane while you harass, fine, go for Tlord... but why is everyone so enamored with this master? Is it just flavor of the month, or does everyone genuinely think it's better in all situations? I have to pose the question, should we all forsake scaling runes? Because it's the same concept. It scales your damage BETTER as the game goes on, giving you late game presence. Is that no longer viable? I mean, I had people telling me that I seriously need to alter my build, because I don't have ThunderLord's Decree.

It's just so weird to see a mastery like this make people go crazy and say it's objectively the best in every way. Does anyone agree? Or am I just going on about nothing?