Of course, like others have pointed out, shared "temp email" services are more likely to be blocked.
Some people like to create dozens (or hundreds) of disposable emails because they just need it for a single purpose (e.g. downloading a pdf whitepaper that's hidden behind an email sign-up). The email address is truly ephemeral and they don't want to create a whole new Gmail/Outlook inbox just for that. There's also UI/UX friction in Gmail/Outlook workflow of multiple screens to create a new inbox instead of being a convenient one-click like the disposable email services.
Also, Google and Microsoft puts a limit on the max # of new email accounts. They don't explicitly publish the max number but it depends on various heuristics such as your residential ip address, country, etc. E.g. some users only got up to 4 gmail accounts tied to a phone number. Others got 10 before being cut off. There are rate limits so one can probably get more accounts if creating them is spread out over multiple days.
Using one-click disposable email services or using your own domain to create unlimited email aliases lets one go beyond the limits of free Gmail/Outlook accounts.