Because volatility can hurt you. Let’s say you have a 1000k portfolio and you need 100k which you take out at the start of each year. For the sake of argument let’s look at scenarios where portfolio gets 100k of income (tax free) at 10% and a scenario where you get 10% as capital gains.
Let’s say at the start of the year, due to market volatility, the portfolio drop so 200k. In the capital gain scenario, you sell 100k are left with 100k, which at the end of the year grows by 10% to 110k.
In the dividend scenario, you get dividends of 100k and no growth so stock is worth 200k at the end of the year.
The next year the stock recovers by 500% and in the gains scenario you have 550k, in the dividend scenario you have 1000k still.