They have to be done now. Otherwise were just reacting to the exponential spread. Why is the closing of museums, large social gatherings not necessary yet? Have you not seen Italian PM Conte addressing Italy that there would be no more gatherings, no more night life, etc and it was already too late.
There are different circumstances in different countries so unless you have access to the data and the modelling they're working off it's impossible to say when exactly each country should implement each measure. That's why even countries that are following the exact same broad principle in terms of delaying certain messures generally (like the UK and ROI) are implementing different measures at different times. It's all context dependent. It's all also dependent on the models they're using and the directions they decide to go in. So direct comparisons between countries are difficult.
In general what you can say though is that these countries are striving for a delayed, flat peak. Which means both reducing the total number of people who might get infected
and ensuring more of those who do get infected do so on either side of that peak.
In other words the UK (for example) needs a certain amount of the population to have become infected, been taken care of and recovered before the peak comes in (according to them) 10-14 weeks. What they don't need is to stop those people from getting infected in the period before the peak, only to then have them add to the peak.
So the answer as to why it's not necessary for them to implement more severe measures now is that, according to their modelling, that won't help spread infections out in the way they're looking to achieve. It would instead result in fewer infections now (when it can be handled) and more infections later (when it can't). It's a difficult balancing act.