It's generally not done because it's a security issue as an attacker could register before you with your email and gain access to your personal info (billing an

It's generally not done because it's a security issue as an attacker could register before you with your email and gain access to your personal info (billing and shipping address and maybe last four of credit card). What would be best if the eCommerce site would send you an email to confirm your email address to unlock your guest checkout orders. That's a fairly complicated flow for most eCommerce outfits so I think they just default to not show guest orders.