Because to a first approximation everyone understands, and can get, email, while maybe 5% of the population, if that, understand RSS feeds. My 80 year old aunt can read email. While I'm sure I could teach her to use RSS feeds, it would not be a task I would look foward to. And she has me. Most people don't have me, or anyone like me (or, likely, you, or the majority of other users of this site).

What you're saying is roughly equivalent to "So what if they can't use the telephone or television? Can't these folks just use Morse code?"

Or they could use email but via someone other than Mailchimp, such as ActiveCampaign, MailerLite, Sendinblue, GetResponse, Moosend, EngageBay, Omnisend, Zoho, Mailjet, ConvertKit, Benchmark, AWeber, HubSpot, Constant Contact, DotDigital, Campaign Monitor, Sender, SendPulse, Klaviyo, Jilt, Autopilot, Drip, Keap, and probably many others.

Email is not like, say, Facebook where to reach someone on Facebook you need to send via Facebook. How you send email is largely irrelevant as long as you pick a provider that doesn't send so much spam that their outgoing mail gets blocked.

> Because to a first approximation everyone understands, and can get, email, while maybe 5% of the population, if that, understand RSS feeds.

At some point in history, 5% or less of the population understood email. RSS feeds are not inherently more complex than email, in fact they're quite a bit simpler. (And even if you don't grok RSS feeds, you'll probably grok a weblog; RSS is only relevant here because it gives the exact same affordances as something like a Mailchimp newsletter, even though it works differently under the hood.)

Other than not coming preinstalled on every operating system and having reply capability, you mean?