Finland

In the early days of internet, when people could host mail servers with their residential connections, there were plenty of people who did not understand how to secure their SMTP server.

That meant, even without those people realising it, their self-hosted mail servers were used as relays for spamming. That's why residential IPs SMTP ports are blocked AFAIK.

I'm not sure even digital rights associations (EFFI in Finland) oppose this. If you want to self-host your mail, you pretty much need static IP address with good reputation anyways to get anything delivered.

which does not explain why this port or the others are blocked. I also lack the technical background to understand this decision.

Don't take this the wrong way, but understanding the reason for that decision is pretty important if you're planning to run your own email server. A misconfigured email server (which is very easy to do) becomes a problem for everyone else when it inevitably gets used to spam. There's also a lot of ancillary things to configure correctly as well (DKIM, SPF, DMARC policies, spam filtering, etc) lest everything seems to work but no one is able to receive mail from you or it always ends up in their spam folder.

While I disagree with port 25 being permanently blocked on residential (and often even business-class) connections, I understand why in the grand scheme of things.

I don't read Finnish, but here are the general reasons why:

1. Port 25 is for SMTP transport and typically only used for server-to-server (MTA) email traffic. This is unauthenticated between servers. Clients (MUAs) connect through a "submission" port which is pretty much expected to be authenticated/access-controlled. That's why you can send emails to an email provider but you can't be an email provider yourself. By blocking port 25, malicious people or people that have been compromised with malware cannot just blindly blast out spam email. This reduces spam considerably, though with a compromise of slightly restricting what a residential connection can be used for.

2. Most big email providers universally block emails that originate from an IP address that's assigned to a residential IP/provider. Same reason as above. This means even if your ISP were to unblock port 25 for you, you likely wouldn't be able to send email to any major email provider (Gmail, Outlook, Yahoo, AOL, etc) as they would just sinkhole any messages you send to users there.

That's pretty much it in a nutshell.

Can you bypass that and host at home?

Yes, if you're willing to work for it. You can setup a VPS (cloud server) and port-forward across a VPN connection to your home server. Your DNS records for your email server would point to the VPS's IP, and the email server would need to be configured to use the VPS as its default route so all traffic goes in/out over the VPN connection. This is how my email server is configured.

Sounds easy enough, right? Well, good luck getting a VPS with a "clean" IP. Most VPSs you can get in public clouds are already on one or more public spam blocklists as well as many private/internal blocklists. You can clean up an IPs reputation and make it work with minimal to no delivery problems, but it's a LOT of work and often requires finding hidden forms to submit the request (Microsoft/Outlook was a brute, and I only found the link to the form in a forum post). I've cleaned up two IPs like that, and it took 2-3 weeks of work before I was able to get reliable delivery.