Many Protestant groups are close together in ways of doctrine so they can easily switch denominations. Especially Methodists, Disciples of Christ, Presbyterians, and the United Church of Christ
With the ecumenical movement have come mergers thus making fewer denominations. The United Brethren merged with the Methodist church to make the United Methodist church. The Congregational Christian Church merged with the Evengelical and Reformed church to make the United Church of Christ, and the E & R was an earlier merger. The UCC is growing close to merging with the Disciples of Christ.
The Lutheran Church in America mereged with the American Lutheran church to make the Evangelical Lutheran. The ELCA has altar and pulpit sharing with the Episcopal church.
The mergers and reunions are mainly as we see fairly limited to Mainline Protestants. It is the more fundamental and Evangelical churches that continue to fracture, over and over.
My Baptist friends tell me that when two Baptists dissagree they just start a new congregation.
In the churches of Christ where I was raised they split at the drop of a hat. I have read there are seven different groups all using the same cofC name that deny the others are Christian. They split over such issues as sunday school or no sunday school, how many cups are used the pass the grape juice in their Communion. The others issues they split over are of equally earth shattering importance.