First off, the Suns split the regular season series with the Warriors 2-2.
That aside, regular season basketball is one thing; PLAYOFF basketball is another - The Nuggets took 3 out of 4 from the Warriors in the regular season, and yet they're in danger of being swept right out of the First Round by that very same team.
How much postseason experience do Devin Booker and the Suns have?
How much postseason experience do Ja Morant and the Grizzlies have?
Now compare their combined experience to that of Steph and the Warriors. They've won as many championships with that core as Booker's Suns and Morant's Grizzlies have won playoff series' COMBINED.
The Suns and Grizzlies THINK they can win a championship, the Warriors KNOW they can. How many Game 7s, Elimination games, and high pressure situations have the Warriors been in since they made their first appearance with that core of Steph, Klay, Draymond, and Iguodala back in 2014? Not only have they won many of those crucial games, they've won some of them on the road. Games 4 and 6 against Cleveland in 2015, Game 6 against OKC in 2016, Games 1 and 7 against Houston in 2018 and Game 6 in 2019 (Two seasons where Houston had Golden State's number in the regular season, yet look what happened in the playoffs.) They've been in those type of situations countless times and they have won more than they have lost. This is the same core and same coaching staff that, when healthy enough to be on the court, went to 5 straight NBA Finals and won 3 of them. They know what it takes to get there and to finish the job.
Phoenix had ONE great run last year where they made it to the Finals and came up short followed by a dominant regular season this year, and Memphis also put together a very strong season and this would be the first time they've made it past the 1st round since 2015 when... they lost to the Warriors... Who went on to win their 1st of 3 championships. That's assuming the Grizzlies get past the 1st round in the first place since the Timberwolves are giving them ALL they can handle.
The Grizzlies, and to a lesser extent, the Suns, are unproven in these major, high pressure, playoff situations. The Warriors have been there and done that, many, MANY times. They were a dynasty in the mid-late '10s with that core they still have. A core that is now back in tact, back together, and getting healthy for the first time in 3 years, and has plenty of support behind them.
The Suns pretty much put-up better numbers all across the board and again they easily won elven more games than the Warriors did during the regular season. Again, if Booker is back by the WCF the Sun will be the favorite team in that series.