Why do I need a subscription just to do simple editing?

You mean, like Microsoft, who want you to pay for Word before you can edit a document? What do you think Adobe are there for? They exist to sell stuff, and there are freebies to get you hooked. The usual deal.

You're not very precise in your statement. Editing a date can mean to edited a date field or a plain text showing a date in a PDF. The latter would require Adobe Acrobat while the other is doable with the free reader. So, for Reader you don't need a subscript at all for Acrobat you can use a subscription but you can also get a perpetual license you can use offline and for ever. It's up to you:

https://www.adobe.com/acrobat/pricing/compare-versions.html