Because the Kremlin wants europe to take an overt action to justify mobilization. There are many better assymetric ways for europe to retaliate.
This compounds because NATO needs to act collectively to effectively act militarily (maybe Poland could go it alone).
Originally Europe didn't want to supply tanks, then it was only short range missiles, then it was longer range missiles as long as they weren't used to attack targets inside Russia - now it's actively supporting doing military strikes deep into Russia, and openly talking about European boots on the ground in Ukraine ( not that they likely didn't have deniable special forces there from the beginning ).
Strategically it's not in Russia's interests at all to have a wider escalation right now - they have their hands full. Any Russia sabotage is a direct response to missiles and drones using Western technology blowing up warehouses and oil installations deep inside Russia - widening the war is not in their interest or their aim here - they are trying to deter that.
There is the wider question of whether Russia's long term goal is to take back the old soviet states bit by bit in a way that doesn't trigger a full response - but I'd argue there is already a clear red line there - and that's the Nato border.
Do they? Why? They're already in a stalemate of a conflict, why would they want more fully-mobilized enemies?
35% pensions (usually also has some health care here)
35% social security and health care
15% education
15% everything else, roads, bridges, government buildings, ... (army is in here @ maybe 5%)
They're also drowning in debt, of the non-tiny, only Germany does better than the US. So they can't realistically borrow to finance this for 5 years. They're trying to cheat, and treat the EU like a separate country (meaning get investors to treat the EU as a separate state that is not on the hook for the debt of member countries but levies it's own taxes, then borrow against that, but EU countries are cheating, for example using money the EU borrowed for military purposes to fund general expenses like fixing roads)
If the EU countries mobilize the army will go to 20-25% of government expenditures easily.
That means either pensions or social security HAVE TO go down 15% (or 7.5% each). That will cause a revolt since a great many people can't survive like that. At that point there will be no choice for a lot of people but to tear the government down.
That may be the plan of course.
1. either their actions are ignored by the victim states, which makes this very cheap and effective sabotage to weaken EU members.
2. or alternatively, if there's military retaliation, putin can finally do the mobilization he promised not to do for his war on ukraine, because he can now assert that the country is under attack by foreign nations.
the EU / the member states are now in a pickle: they know it's russia but can't say it out loud because then they'd have to take action. they also can't do nothing because then russian sabotage could wreak havoc with the population and economy. so what can they do?
1. military support of ukraine 2. further sanctions of the russian economy, which includes getting rid of dependencies on russian goods 3. re-arming and modernizing their own military (i.e. introducing modern drone warfare capability, launching their own satelite intelligence, ...) 4. start opposing/controlling russian influence by local malicious actors, i.e. russia friendly right wing political parties (which is somewhat complicated but not impossible in a democratic setting)