Simple answer - Money
Your progress will be dramatically slower if you don't grind arena.The 4 star basic arena twice a week gives out in milestones:135 units.14000 phc shards13500 battle crystals23000 goldRank rewards: 500 four * shards. 200 five * shardsTimes that by two.270 units28000 phc shards27000 battle crystals.46000 goldYou also would get the 11-25% rank rewards most weeks giving an additional 500 four star shards and 200 five star shards Again you can get that twice.This also doesn't account for the battle crystals for the individual fights and the gold which adds up a ton.That's just one arena.You want a deep roster where someday you get lots of Max sig crystals you have to spend tons of money or grind arena.
What’s your roster like? I find getting a heap on 4* r3 champs helps get all rewards in 4* basic arena.Not a keen grinder but as others say, it’s a necessary evil, particularly when you need carb cores when doing ftp.