For $400 (assuming that is USD) total I would buy these.

https://www.musicdirect.com/equipment/speakers/wharfedale-diamond-225-bookshelf-speakers-pair

https://www.amazon.com/Audio-Amplifier-Receiver-Integrated-Speakers/dp/B076P2VS9H

https://www.amazon.com/Pyle-Phono-Turntable-Preamp-Preamplifier/dp/B004HJ1TTQ

Or if you want a speaker alternative which will have a more neutral presentation (probably one toe on the brighter side of neutral I would guess) rather than the warmer and more laid back sound of the Wharfedales, consider these which are on sale.

https://www.amazon.com/Polk-Signature-Elite-Bookshelf-Speaker/dp/B09B7XF3Z9

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Can anyone also explain why a receiver/passive speaker set-up would be needed instead of powered speakers?

There's no reason why you would need to go that route... powered speakers would do the job. But you might want to go that route because the selection of passive speakers is much bigger than the selection of powered speakers. With a wider selection comes more competition, and potentially better price/performance as a result since you have companies trying to undercut each other. In your price range in particular I don't think any powered option can compete with the better passive options at the same price.

Once you hit $550+ you kind of reach the next tier of powered speakers (inclusing speakers that use active DSP tuning such as the Vanatoo T1 Encore and the PSB Alpha AM3) and passive vs. powered becomes a more interesting choice.