Date cast format not being applied

A model (in Laravel 8.49) contains:

protected $dateFormat = 'Y-m-d';
protected $casts = [
    'service_start' => 'date:Y-m-d',
];

and the service_start field is defined as a date in the MySQL DB.

Yet in a controller that receives a service_start param containing 2021-06-29 00:00:00 (the serialization of Carbon::today()), the format makes it all the way through to the model during a create, resulting in an error:

ERROR: SQLSTATE[22007]: Invalid datetime format: 1292 Incorrect date value: '2021-06-29T00:00:00.000000Z' for column 'service_start' at row 1

Give that this datetime format in the query is clearly different to what was in the request, something is processing it to give it that format.

I see lots of references to the $dates property, but that no longer seems to be documented so I gather it's obsolete (I assume replaced by $casts) – but I don't know for sure.

I've also seen mention of using a mutator like this:

public function setServiceStartAttribute($date): void
{
    $this->attributes['service_start'] = Carbon::parse($date);
}

Why would that be necessary? Isn't this what the cast does anyway?

Why isn't this date being cast according to the format it's been given?