Why do Grandmasters play different openings but engines not? Why do they change openings if engines only like one?
Engines

Engines usually have opening databases. Every variant in each opening has a pre-calculated score and according to the opponent's move, engines will choose highest scored move. So, sometimes you may feel they play as same as previous games.

But, that's not all, many good engines will choose a random move among a group of best candidate moves because their programmers don't like biased engines.

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Grandmasters

On the other hand, grandmasters, they know lots of openings and they're incredibly skilled on some of them. Sometimes they persist on a specific opening to learning more. Sometimes they play different openings against opponents to make them surprised or confused.

Head-to-head matches between two grandmasters usually lead to a little number of openings. Because the white player tries to choose an opening based on his power and opponent's weakness.