Why can't existence be mostly good?

nibbanam paramam sukham - Nibbana is the greatest bliss.

Dhammapada Verse 204 Pasenadikosala Vatthu

Ultimate happiness is Nibbana. This is beyond existence. If at all, the best analogy is infinity or singularity than being zero, where mundane experiences are comparable to finite numbers. Finite numbers, however big (even if you reach the Brahma Planes), whatever end of the pleasure meter, is always dwarfed by infinity. Pleasure of satisfactoriness at lower states do not stack against the satisfactoriness of Nibbana, abite all conventionality breaks down, hence what ever conventional is not present in Nibbana. Normally the pleasantness arise out of sensations or feeling, which are also conditioned, but in Nibhana this is not the case.

If you were certain about enlightenment you would either be wrong or enlightened.

Your thoughts sounds like mine were in the beginning. I thought "What's the big deal, I don't suffer.", but OMG do we ever suffer and we don't even realize it!

It is a subtle thing to realize and there is nothing good or bad about it. Good or bad are words made up by human beings. The Dhamma is about moving past words that we think actually exist in our hearts when they only exist in our intellectual and story making minds.

Remember the world. You can see how we all suffer rather clearly, if you look at the world as a whole.

Enlightenment & the path to enlightenment (such as meditative jhana) is a higher & better pleasure than any pleasure that can be experienced in ordinary life.

This is the simple, accurate & correct way to view enlightenment.

Please refer to this link.

65. Bhikkhus, there are two kinds of pleasantness. What two? Household pleasantness and the pleasantness of one gone forth. Of these two, the pleasantness of one gone forth is better.

66. Bhikkhus, there are two kinds of pleasantness. What two? Sensual pleasantness and the pleasantness of giving up sensuality. Of these two, the pleasantness of giving up sensuality is better.

67. Bhikkhus, there are two kinds of pleasantness. What two? The pleasantness with substratum and the pleasantness without substratum. Of these two, the pleasantness without substratum is better.

68. Bhikkhus, there are two kinds of pleasantness. What two? The pleasantness with desires and the pleasantness without desires. Of these two, the pleasantness without desires is better.

69. Bhikkhus, there are two kinds of pleasantness. What two? Material pleasantness and immaterial pleasantness. Of these two, immaterial pleasantness is better.

70. Bhikkhus, there are two kinds of pleasantness. What two? The noble pleasantness and the ignoble pleasantness. Of these two, the noble pleasantness is better.

71. Bhikkhus, there are two kinds of pleasantness. What two? Bodily pleasantness and mental pleasantness. Of these two, mental pleasantness is better.

72. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness with joy and pleasantness without joy. Of these two, pleasantness without joy is better.

73. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness on account of agreeability and pleasantness on account of equanimity. Of these two, pleasantness on account of equanimity is better.

74. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness on account of concentration and pleasantness on account of distraction. Of these two, pleasantness on account of concentration is better.

75. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness with joy for an object and pleasantness without joy for an object. Of these two, pleasantness without joy for an object is better.

    76. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness on account of an agreeāble object and pleasantness on accout of equanimity for an object . Of these two, pleasantness on account of equanimity for an object, is better.

76. Bhikkhus, there are two kinds of pleasantness. What two? Pleasantness born on account of a material object and pleasantness born, on account of an immaterial object. Of these two, pleasantness born, on account of an immaterial object is better.

As for your other concerns, they are unrelated to Buddhism. Buddhism is not for people who believe ordinary life can be pleasurable most of the time. Buddhism never ever states all people must follow the Buddhist way of life.

"imagine if we were to live a life composed of 99% satisfactory states"

Then that would just be an imagination. Because 100% of life(moments of arising and ceasing of five aggregates) is composed of unsatisfactory states.

This is why the first noble truth is something that you should realise. The noble truth of suffering is not just about painful sensations. That is just one type of suffering called Dukkha-dukkha. There's also Viparinama Dukkha that is caused when you cling to pleasant experiences. Then there is Sankhara Dukkha which means the unsatisfactory nature of all conditioned phenomena.

You think drinking, eating, partying or simply feeling pleasurable sensations is good because you haven't realised the first noble truth yet. You think Nibbana is zero because you haven't realised the 3rd noble truth yet.