Why don't I just serve https only?
The main reasons are the default behavior of browsers and backward compatibility.
When an end-user (i.e, without knowledge in protocols or security) types the website address in its browser, the browser uses by default HTTP. See this question for more information about why browsers are choosing this behavior.
Thus, it is likely that users will not be able to access your website.
It is possible that some users with old systems and old browsers do not support HTTPS or more likely, do not have an up-to-date database of root certificates, or do not support some protocols.
In that case, they either will not be able to access the website or will have a security warning. You need to define whether the security of your end-users is important enough to force HTTPS.
Many websites still listen to HTTP but automatically redirects to HTTPS and ignore users with really old browsers.
could someone spoof http://www.example.com if I don't set up HSTS?
If an attacker wants to spoof
http://www.example.com, it needs to take control of the domain or take control of the IP address in some way.
I assume you meant: could an attacker perform a man-in-the-middle attack?
In that case yes, but even with or without HSTS:
Without HSTS: An attacker can easily be in the middle of your server and the user, and be active (i.e, modify the content) or passive (i.e., eavesdrop)
With HSTS: The first time a user try to visit the site using HTTP, an attacker could force the user to use HTTP. However, the attacker has a limited time window of when it can perform its attack.
What you should do?
Like many websites, you should allow HTTP connections and make you server redirects the user to the HTTPS version. This way you override the default behavior of browsers and ensure your users use the HTTPS version.
Old systems without the proper protocols or root certificates will not be able to access the site (or at least will have a warning), but depending on your user base this should not be an issue.
It will do more harm than good to disable HTTP. It does not really provide more security.
Any security added to protect a resource is useless if it prevents most of its users from accessing it. If your end-users cannot access your website because their browser default to HTTP and you do not listen for HTTP connections, what is the benefit?
Just perform the HTTP 301 redirection to the HTTPS version.