Because that is how reality is. And an RPG is a second "reality".
And having a system of; fail 3 times in a row, then you get 100% success on your 4th. It makes no sense when it comes to an RPG.
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The RNG is not an entity nor anything with a semblance of thought. It doesn't hate people, it just does what it's been programmed to do. Unless the probability was a 100% chance of success every meld, then someone will be the unlucky person. Having a few unlucky person does not justify removing any sense of risk from the system.
A lot of things in RPGs get explained away to make sense, like Tokens. Say you fail a meld with your materia on your scroll and all that is left is an untradeable unsellable powder of the materia you tried to meld - by using that powder in conjunction with a meld of the same type you get a boosted chance (if SE feels nice - 100%, if SE wants to torture us but still give hope - say something like 30% and you get two powders if it fails again).
The reason why I say of the same type and tier is because it would be pretty unfair to fail a useless tier I to then put in a great tier IV.
Defending Novus meld failure is like defending getting kicked in the nuts. There's a possibility you'll never get kicked in the nuts on your way to work and everything will be great, or there's the chance you could get kicked in the nuts 10 times before you get there. The question is, why would you want to put in the chance of getting kicked in the nuts at all? The people that defend it were never kicked in the nuts or had expensive nut-kick cushioning undies on that other people can't afford and then say "I don't see how getting kicked in nuts is such a big deal."
You, like many others, are making a very fundamental mistake. You are trying to value a novus weapon. A novus weapon will only cost as much money as the player is willing to spend on it. Which can range from 0 to upwards of millions of gil. Or if you feel you need to spend gil for materia, you can at least break even by selling unwanted materia obtained from spirit bonding.
And secondary stats will continue to have a much smaller impact to overall effectiveness compared to the sheer power of primary stats and weapon damage.
Sure, two stats may represent the 'optimal', but most people can show you the loss of melding three stats is practically negligible.
Honestly, now that the nexus step is out, I think people are probably better off just getting to that step ASAP as the gain in weapon damage in primary stats outweighs trying to get a future more points in secondaries with Teir IV (yes I realize your secondary choices scale too, but the same principle applies as it does at the Novus state).