It is not an arbitrary decision.

For one Safari compiles block lists to perform better, but it can be noticed at startup for big lists.

Then there is just resource constraints since the focus is mobile. Chrome on mobile notably supports no extensions.

But I do wish desktop Safari was more lenient.

Chrome's decision is also entirely arbitrary, so it's not a great example. Firefox on mobile notable supports Chrome extensions, without any real issues or battery drain whatsoever.