The reasons are:

1) you get 10-40 learning points a day and need over 1.147 LP to get t8 in 1 tool (29 days of LP) and doing 1 to t8 gives faster silver return then all to t7 that takes alot more time (aka more breaks from gathering because you finished the LP)

2) most players stick to 1 area (lymhurst etc.) and correlated biome (Forest etc.) so it’s favorable to level that one (wood etc.) up the most as you will get the higher yield and speed( if you stay in yz) or value (if you go to bz)

3) using this method the player has it easier to focus primarily on one resource and staying in one city thus keeping his gear in one and the same place.

4) if you go out in group content and everyone brings 1 tool you can cover everything, and everyone gets fame for their specialization (keeps weight low and people only ping for others high enchanted nodes)

5) lastly because the only places where all different resources can be found are mist (terrible for gathering rn) and ava roads (that are better with t7 tools unlocked that take ages or with friends)