I guess because according to them they think it will be bad for business. Ultimately thats one dimension where nobody can know until it happens, the only thing you can go on is who has moved away/ is moving. To that end in 'my area', financial services, I do know that since the vote 29 financial services companies have already moved from UK to Dublin, and its predicted about 7,000 jobs will go from UK to Dublin. Dublin is highest in the 'relocation' league (Barclays alone put over £2bn into Dublin as opposed to London), followed by Luxembourg (25 companies) and Frankfurt (24). It was genuinely a field day for EU countries picking up UK offices of major companies, I've even seen 'hit lists' of companies that are targeted to get to move away from UK.
edit- and I chuckle when all the companies make announcements and immediately there is a huge chorus of "yes but its nothing to do with brexit"- brexit won, its going to happen, own it & accept the implications, and move on with doing something to avoid loads of people losing their jobs instead of pointing fingers- and that does not involve spending millions on coins, teatowels and blue passports.