Storing data on Excel spreadsheets is a cheap way of running your business. While cutting some corners on cost can be efficient, investing in a more secure bespoke software is better in the long run. One of the security issues involved with using Excel is that the changes made to the spreadsheet are not tracked. This means that effectively anyone that gains access to the system can change any amount of figures without consequence. You will need to spot these changes ASAP before they start snowballing other figures the wrong way. If someone with ill intentions gained access, they would be able to do possibly irreparable damage to your data.
You may want to track who is doing what inside the business. If a team has control over different areas of the spreadsheet, you want to ensure everyone is working. When working with spreadsheets, you may need to work on different versions and then merge them which can be a pain without changes being tracked. With bespoke software, everyone can work on the same database at the same time and can use it on any computer.