Learn Dutch: De realiteit is nou eenmaal simpel.

It expresses inevitability or resignation. While De realiteit is simpel is a neutral statement of fact ('Reality is simple'), adding nou eenmaal implies 'That is just the way it is, and we cannot change it.' It gives the sentence a tone of finality or acceptance.

The translation uses 'simply', but does not gewoon also mean 'simply'? What is the difference here?

Yes, gewoon means 'just' or 'simply' in the sense of something being straightforward (De realiteit is gewoon simpel). However, nou eenmaal adds a layer of unchangeable fact. You use nou eenmaal to express that something is an absolute given, often when dealing with a situation that cannot be altered.

Could I use de werkelijkheid instead of de realiteit in this sentence?

Absolutely. They are often used interchangeably to mean 'reality'. De werkelijkheid focuses a bit more on what is factually actual, while de realiteit often refers to the practical circumstances you have to deal with. In this context, both work perfectly.

Why does nou eenmaal come right after the verb is?

Modal particles like nou eenmaal typically sit in the middle of a Dutch sentence. They come after the main conjugated verb (is) and any pronouns, but before the adjective they modify (simpel). Putting it at the very end of the sentence would sound unnatural.