There are many reasons why Squarespace is so bad. One reason is that it is very limited in terms of design and customization.
You can only use the templates that they provide and you cannot change much about them. This means that your website will probably look very similar to other Squarespace websites.
Another reason why Squarespace is so bad is that it is very expensive. Their cheapest plan is $12 per month and this only gives you basic features.
If you want to use any of their advanced features, you have to pay even more money. For example, their ecommerce plans start at $26 per month. This is a lot of money to pay for a website builder that has so many limitations.
Finally, Squarespace is not a very user-friendly platform. It can be difficult to figure out how to do things, and their customer support is not great. There have been many reports of people waiting days or even weeks for a response from customer support.
All of these reasons make Squarespace a very bad choice for anyone who wants to build a website. There are much better options out there, such as WordPress, that are more affordable and easier to use.