Because, for mortal races, evil is cultural not in-born; individual exceptions, through fluke of circumstance and opportunity, can and do exist, and it's important that they are allowed to.
That's where our opinions differ, I take a deterministic approach and therefore believe that monsters are inherently evil without any choice.
You're free and welcome to have a preference in your fantasy for how you'd like to imagine things... but as long as we're talking about D&D in the established realm spaces that we currently have, I'm sorry, but you're just simply incorrect here. There's no subjective opinion about it – there is simply a fact of how the realms are set up, and you're wrong. You can set up your world that way at your own table, absolutely – but canonically, that's not how it is.
I'll note again that you still didn't actually give a 'reason' after the bold and strong claim that they were that way for a reason... what is the reason you insisted existed before?
As I said though... understanding this does not actually run afoul of the problem that you yourself seem to be a good example of; you like and want simple black and white to fight, and that's fine, that's great... mortal races don't need to be inherently evil for that to still be a comfortable space you can occupy, though, because the evil in mortal races in cultural, and inherent also still evil exists beyond that in other entities across the planes and even on the material.
The important point to be made is that this doesn't risk the 'we still need evil things to fight' problem
Is that a problem though? That we need "evil" things to fight? Can't our opponents simply have another perspective than ourselves without being tagged evil for the race or culture they come from?
It does become a problem if taken too far. Yes, it can be exciting and interesting to work with the myriad of grey-shaded morality and deal with difficult questions of right and wrong, morality and ethical action, and all of that... but if
every conflict becomes a moral quandary, that very quickly stops being fun for most players. We still need 'simple' evil, that's black and white and easy to fight, without internal conflict or greyness.
Remember that evil in the realms is nuanced; it is defined predominately by
Selfishness. Evil characters put their own needs ahead of others, and give less value to the needs or rights of others; they view their goals, their rights, their safety and their property as more important, more valuable and more worthwhile than other peoples'. This is the core outlook that creates the knock on effect that we see creating the myriad different types of evil action that villains tend to take. An evil society is one that is underpinned by a similar destructive selfishness – a society that is internally ruthless, and which values other peoples and other societies less than their own; generally one which places little value on the lives, comforts or safeties of its individual members, save as behind what they can secure for themselves at the cost of others, to get ahead.
Yuan-ti society is an evil society, and yuan-ti who live within it, shape their lives by it, and view it as the 'right' way to exist, are by extension evil characters – but they are not 'inherently' evil, and individuals that buck this social construct can and do exist. Most don't live long unless they get out.
Just to be clear...
I still agree with you that a main character Yuan-ti pureblood would face prejudice and fear from many populations based on history etc but I don't agree with you that the fear and hate based on prejudice or general conception of a race is a crucial element.
I did not say they were. Please do not characterise me as saying that.
I said hesitation, trepidation and mistrust; caution and reservation based on known elements of the majority of that society from which they come, even if they are no longer a part of it. Decent people don't hate irrationally, and decent people would not... but they could be expected to show mistrust initially, hesitation, trepidation and caution that is motivated by the evidence known of every other member of that culture that they have encountered or known of previously – and again, as I said, this is not an everyone-always thing, this is just a common occurrence; off-setting it, you will have many who will simply take at face value that a yuan-ti apart from her people and working as an adventurer with others is different, and will treat them just as fairly as they would anyone else... but it won't always be the case, and it shouldn't be. The fight to redefine other people's understanding, and to show them who you are, different from who they worry you may be, is and should remain an element that players of monstrous races work with – and how they, as characters, deal with that is a juicy bit of personal definition and development.
=
If you want inherent evil, the realms has plenty of that too; fiends and celestials are the most prominent example, but numerous other entities and creature types exist that have external, objective, good or evil baked into their very essence. Despite what some have said about Wizard's latest actions, that aspect isn't going anywhere. There exists it the realms tangible, objective, very real good and evil as literal things... it's just that mortal races and individual members of mortal sapient creatures are not considered to be in that bundle any more.
