The look and user experience is bad when there are too many terms for custom fields.
It would be better if it was a sliding and searchable area. Please see the  picture.

@arif
Isn't simpler to reduce number of terms to acceptable amount?
Custom fields should use standard select box.

I don't want to enter too many terms.  But I'm trying to make a used toy site.  I want to filter by toy brand.  There are so many toy brands.  I looked at the shopping and advertisement sites in my country.  They all use filtering as in the picture I posted.  Maybe it can be done by special request in the future.  Thank you for your interest.

Yes that's possible, but they most probably paid to some developer to do such filtering or bought ready-made solution that contained it already.
But it's still custom design that get oldish after some time and will be replaced with something else, would cause never ending troubles to existing themes that does not expect such filtering etc etc.
So it does not make sense to build such feature in core.