Saber Vs Salter

I have no idea where those scores came from or how the scoring system works, but as far as I know, the reason salter is considered "better" is that Salter is a 4*, so: easier to obtain higher NP levels, lower party cost, and for her rarity, deals more damage with her NP.

Plus, Salter's attack actually isn't actually that much lower than Artoria's, and without factoring in attack/defence buffs, salter's NP actually does MORE damage than Artoria's (17293.5 vs. 12623.6). Even including the difference in charisma levels between the two, Salter's NP damage is still higher.

(Source for damage calculation: https://gamefaqs.gamespot.com/boards/180151-fate-grand-order/76036049)

If you're talking about the tier list, the tier list on the 5* list does state that every 5* Servant is worth leveling even in the bottom tiers due to their general performance and stats being better than 4* and below overall. The tier list is separated for a reason, in that it's comparing them to servants of their own rarity and not with the others.

Artoria is lacking compared to her fellow 5* servants, but she does still have better base stats than her 4* counterparts including Salter.

It's mainly because, as a 4* Salter should be easier to attain, at least when she is on rate up (Story-Locked normally). That said I have normal Artoria and not Salter, so whether or not she is actually easier to obtain is up for debate.

But the biggest thing is that Salter's NP has a unique 450% modifier on it, meaning that, as long as their NP levels are the same, Salter always has a better NP.

Also something worth noting is that sticking 2 grails into Salter to get her to 90 leaves her with a higher attack stat than the OG,

Dont worry salter will fall later on unless it for farming

Problem with vanilla Artoria is she has more of her stats pumped into her HP rather than attack while the opposite is true for her alter form. So while her charisma skill is lower rank than OG Saber it balances out damage wise. Couple that with the 4 star status having a 4% Drop rate makes getting a NP2 Version of her more likely than OG Saber. She might get a costume buff in the far future like Mashu giving her Avalon but its all rumors and speculation right now

One of the reasons why Salter is a lot of times considered better is because the NP. If you look at the stat closely, Saber's NP damage at Lv. 1 is just 300%, after interlude it will be 400%. On the other hand, Salter is at 450%. Therefore, even with Saber being 5 star, their NP will deal similar damage.
Second, Saber's growth curve is linear while, Salter is S, that means at middle levels, Salter's would be on par if not better than Saber. The difference don't show until after Lv. 80 ish, even then I can't say it's significant.
The other reasons, as everyone have said, it's easier to level up NP for 4 star servants.