It's all about cost.
The MLB pay system and tax system has created a situation where the cost of your talent can rise by multiples in a short time without you doing anything.
2018 AAV's:
Chris Sale 12.5
Mookie Betts 10.5
Xander Bogaerts 7.05
JBJ 6.1
E-Rod 2.375
Total 38.525
Avg 7.705
2020 AAV's including arb projections:
Chris Sale 25.6
Mookie Betts 27.5
Xander Bogaerts 20
JBJ 11.5
E-Rod 9
Total 93.6
Avg 18.72
So for these 5 players, the cost is 250% as high 2 years later.
And the initial luxury tax rate is also 250% as high (from 20 to 50).
It's a system that makes keeping a good team together for even a short period very difficult, no matter how rich or smart you are or how much planning you do.