There are several reasons why/that ...

[tzfujimino]

We do not use reason in the plural with a that-clause:

There are several reasons why I don’t like the book.

Not: [STRIKE]There are several reasons that …[/STRIKE]

Source:
https://www.google.com/amp/s/dictionary.cambridge.org/ja/amp/british-grammar/reason

Do native speakers agree?

 

[5jj]

I don't.