Because if GOD is not already WITHIN the person HE IS NOWHERE.
All that bogus claptrap notwithstanding there are more phonies that profess a belief in GOD than there are grains of sand on all the beaches of the world. Phonies and fakes and charlatans and con artists and scammers and shameful blasphemous scoundrels. GOD IS OMNIPRESENT. Maybe they need to look up what that word means exactly. OMNIPRESENT means EVERYWHERE ALL THE TIME. Fakes phonies braggarts liars the lot of them.
Many people know of God but they don't know God. The Pharisees knew of God, but they didn't know Him - even when He stood right in front of them.
In Jesus' time, the Sabbath was a very holy day. God had explicitly said it was to be a day of rest. Yet, how many times did Jesus confront the Pharisees about the Sabbath? Was it lawful for Jesus to heal a person on the sabbath? Was it lawful for his disciples to pick grain to eat when they were hungry on the sabbath? As Jesus said, people follow the "law" but understand its meaning. God has but 1 law, and that's to love. Those who insist that they must gather together during a time when its not advisable - not due to government persecution, but due to health reasons - are only showing that they think they're more important than anyone else...including God.
These people need to read what Jesus said very carefully...
“Not everyone who calls out to me, ‘Lord! Lord!’ will enter the Kingdom of Heaven. Only those who actually do the will of my Father in heaven will enter. Many will say to me on that day, ‘Lord! Lord! We prophesied in your name and cast out demons in your name and performed many miracles in your name.’ But I will reply, ‘I never knew you. Get away from me, you who break God’s laws.’”
If one doesn't truly "know" God, He won't "know" them either.