The A-record alias you create has to be the same as the name of the bucket, because virtual hosting of buckets in S3 requires that the
Host: header sent by the browser match the bucket name. There's not really another practical way in which virtual hosting of buckets could be accomplished... the bucket has to be identified by some mechanism, and that mechanism is the http headers.
In order to create an alias to a bucket inside the "example.com" domain, the bucket name is going to have to also be a hostname you can legally declare within that domain... the Route 53 A-Record "testbucket.example.com," for example, can only be aliased to a bucket called "testbucket.example.com" ... and no other bucket.
In your question, you're breaking this constraint... but you can only create an alias to a bucket named "simples3websitetest.com" inside of (and at the apex of) the "simples3websitetest.com" domain.
This is by design, and not exactly a limitation of Route 53 nor of S3. They're only preventing you from doing something that can't possibly work. Web servers are unaware of any aliasing or CNAMEs or anything else done in the DNS -- they only receive the original hostname that the browser believes it is trying to connect to, in the http headers sent by the browser ... and S3 uses this information to identify the name of the bucket to which the virtual hosted request applies.
Amazon S3 requires that you give your bucket the same name as your domain. This is so that Amazon S3 can properly resolve the host headers sent by web browsers when a user requests content from your website. Therefore, we recommend that you create your buckets for your website in Amazon S3 before you pay to register your domain name.
Note, however, that this restriction only applies when you are not using CloudFront in front of your bucket.
With CloudFront, there is more flexibility, because the
Host: header can be rewritten (by CloudFront itself) before the request is passed through to S3. You configure the "origin host" in your CloudFront distribution as
your-bucket.s3-website-xx-yyyy-n.amazonaws.com where xx-yyyy-n is the AWS region of S3 where your bucket was created. This endpoint is shown in the S3 console for each bucket.