I am currently a member of a Presbyterian Church, but my family has been a member of a Methodist church, as well as an international non-denomination Christian church while living overseas. I have also attended a couple of catholic sermons, including one in a beautiful church that had been standing since before the second world war. The beauty of some catholic churches is appealing, but I am sure that is not what makes Mass Holy for Catholics. Stained glass and old wood is in no way more Holy than pane windows and plastic chairs.
Most Protestants view a church as a way to grow in ones faith and bring oneself closer to god. Most Protestants believe that anyone can reach God without a middle man. A minister is like a mutual acquaintance of God and his or her congregation, introducing the latter to the former. They then can all hang out together so that the member of the congregation feels more comfortable around God, but after that God’s like, “Hey call me any time. I’ve always liked (well, loved) you.” And so you can call on God anywhere and tell him your troubles and confess when you’ve been a bad friend, or you can thank him for his bountiful gifts and blessings because God always knows exactly what you needed before you even could ask him. It is a pretty sweet set up, if you ask me.
In this way, a church can be anywhere, prayer can be anywhere, you can worship god in any time of the day. A church service is another way to further your relationship with Jesus. It is beautiful because it is filled with people who come together to get excited about a God who lives and loves. Some of my favorite services when I was a methodist weren’t even in a building. We used to have spring and summer services on our pastors front lawn. It was a beautiful way to worship.
On a side note, your question is a logical fallacy. For a protestant to believe in/notice the holiness of a catholic mass, they would have to have the mindset that a Catholic mass is holy, and therefore would already be Catholic. They are Protestant most likely because they believe in Protestant principals and attend Protestant service that they believe to be holy. You get what I am saying? You’re argument is routed in the fact that a Catholic Mass is Holy/the Holiest of all Christian services, and if you believe that, then you are already Catholic. You might call it begging the question, assuming the point, or “petitio principii”, if you speak latin.
And Jesus was Jewish. He didn’t invent the structure of the Catholic church, so we don’t know which way is right. But when it all boils down to it, we all love Jesus, and that is more important than the way we do it.